Who this post is for? Someone who either:

• Wonders if they should start lifting weights, and could be convinced of doing so.
• Wants to lift weights, and doesn't know where to begin. If this is you, you can skip this first section, though I'm guessing you don't know all the benefits yet.

The WHY

Benefits of ANY EXERCISE:

• Great mental benefits. I personally have ADHD and lifting gives me an attention boost similar to my Adderall. I'm not talking about the long term abstract of being happy that you are healthy. Post exercise your mood and cognitive hability will probably me accutely better.
• Improved sleep quality, which will then improve about a million things
• Strongly improved health markers, both blood levels and your body composition. Most people probably would be healthier with more muscle and less fat.
• It fits the hobby-shaped whole your life might have. Something you do divorced from work, with progression, community, and so forth

Benefits of LIFTING compared to other exercise:

• In spite of most of the lies you will hear, most exercise won't build muscle. If you are a fat guy and start jogging or playing soccer, you'll probably actually lose muscle mass along with your fat loss. Athletes that play sports professionally are both genetically gifted, and probably lift weights to support their performance. But why do you want more muscle mass? Benefits in order
  • You'll be more attractive to others and to yourself. You'll probably largely hedonically adapt to seeing yourself look better, as will a long term partner. But you don't hedonically adapt to being treated better by strangers and to having access to more options in dating, if you are interested in dating. 
    Images of outlier physiques you can't probably even reach aside, more muscle is as close to universally attractive as anything you can get.
    The number of times someone who didn't take performance enhancing drugs was described by anyone as "too muscular" in the entire history of humanity can be probably counted in one hand.
  • More muscle and less fat again, benefit your healthy greatly. The latter is easy, the former not so much. If you're interested in quantified self, I'd suggest getting your blood markers before your journey to see what lifting does, including your A1C.
• You might really enjoy it. I personally do. Lifting is a really boring sport to watch, which I believe leads to it being tried by less people compared to good expectator sports like basketball. Lifting has recently really upticked in popularity, and you'll know in two months of doing it if your personality lends itself to enjoying it. Compared to most others sports, measuring progress is easy, feedback loops are not too long, making it an easy habit to reinforce.
• It's a a very easy habit to maintain. You can find a gym almost wherever you move, your office might already have one, and you need no one else to do it with. Weather has no impact on your ability to do it. Compare this to basketball or running outside. Having something keep you from your exercise for two weeks can be devastating for your hard built habit.
• You'll be much stronger in daily live. I get a little jolt of joy when I can do the rare physically demanding task.
• You'll be more resistant to injuries. Lifting doesn't just strengthen your muscles, but also your bones and connective tissue. And compared to most other sports, injury rates in lifting are really low. Lifting will make you age much more gracefully, strongly delay the point in which you phyisically need assistance for things like stairs, groceries and so on. The increased bone and joint health will also come in handy then.

Costs of LIFTING:

• Time. But my prescription is two to three sessions of 45 minutes per week. Marginal cost is zero if you "have to exercise anyway".
• It will take between 2 and 8 weeks of active discipline to stick to it, like most habits. Forcing yourself until it's a habit is a cost.
• Like all new things, you have to learn it. That's where this post comes in.
• You might not like it. You'll feel unconfortable in the gym at first, and for a very small fraction of people that never fully goes away.

My Prescription for those who want to start lifting:

The tacit advice

Not all you need to know is just the program. I will at the end of this both give you a program, and a framework to make many more programs. But first I need to go through an infodump with all the basic things I either get asked / have to correct in people. Each of them is formatted as my suggestion, then the explanation in deepening levels. Feel free to skip as much of the explanations as you won't, unless you disagree with the prescription.

1. You should train your full body every time you go to the gym.
   
   Why? This is called your "split", other common splits being Push-Pull-Legs or Upper-Lower. When you go to the gym, you initiate a signaling cascade that tells your tissues to grow, but you also damage your muscles and tendons and deplete various resources which take time to replenish. This is why you do lifting then rest, and won't get much jacked by just starting to do curls and never stopping.
   The time course of both these processes (recovery and adaptation), however, will not require you to rest a whole week after squats before squatting again. If you go to the gym 3 times per week, on a full body split each muscle gets trained 3 times per week, on a Push-Pull-Legs, only once. Direct research has shown higher frequency (times you train a muscle / week) is better
    
2. If you have less than 6 months of consistent lifting under your belt, I strongly advise you against lifting more than 3 times per week. The extra marginal gains are not worth the extra hassle, and the increased chance of you burning out on it.
    
3. Whenever performing an exercise, your first priority is learning the "good technique". Then do as many reps as you can until you think you can't keep good technique on the next rep. Youtube search the exercise's name, followed by one of the following keywords, and you should get a better than default explanation: "Renaissance periodization", "Jeff Nippard", "Max Euceda". If it is one of Squat, Benchpress or Deadlift, add "Starting strength 5 minutes"
   
   Why?: "Good technique" is on the extreme margins not universally agreed between people who are really into fitness. This is not the type of technique disagreement I'm instructing worried about. Beginners who have never been taught willl often make mistakes everyone agrees are bad, significantly harming results, and will improve from almost anyone instructing them. Good technique is defined as achieving some mix of the following goals, ordered by how much I prioritize them:
   1. Repeatable. You only know that you got stronger in the exercise if you're doing the same thing set to set, week to week.
   2. Ensures you can push hard, via being stable, and using the proper range of motion.
   3. Takes the already very small injury chance and makes it even smaller.
       
4. Ignore anything fancy. Your lifting should look like this: For a given exercise, select a load that allows you to perform between 5 and 20 repetitions, and do close to as many as you can (This bout of continuous repetitions, or reps, is called a set). Rest for a little bit, then do another set. 
   
   Why?: The list of fads that range from useless to beginners all the way to provably worse is long, here are many: Resistance band workouts, "circuits", Special loading techniques like dropsets, specific warmup exercises, mobility work, stretching, activation drills, foam rolling, kettlebells, weird timing (resting too little) and so on*
    
5. Be Not Afraid: Lifting weights, statistically, is extremelly safe compared to any other sport. In the same vein as above, if someone tells you "you need to do X on top of your lifting to make sure you don't get injured", they're probably wrong. If they say "This exercise is very injurious" they might be at best stretching the truth.
   
   Why? : The median strength training practicioner is a 30yo dude, who didn't think about it much at all, and goes to the weight swinging around as much weight as he can. That guy, in all of our data collection, is doing totally fine compared to soccer players, joggers, gymnasts, basketball players, etc.
   The forces you put on your connective tissues in the gym are just not that big compared to the sudden spikes when doing things like jumping or tripping. When professional natural bodybuilders (who are much more risk-prone then you) get injured, it's not something like "I ruined my back", it's usually "I couldn't train that body for a month because of it!!".
   See profesisonal athletes when they retire. The former (natural) bodybuilders have a lot more joint health than the "real sport" pros.
   You don't need a belt, knees over toes is good, upright rows are good. Anything that doesn't hurt your joint every time you do it, will not be secretly harming you in the background
   
    
6. Don't complicate your eating. Try to eat something resembling protein on every meal, don't go beyond that. Don't start meal prepping, don't count your calories, don't change the foods you eat too much. If you're vegan and hate tofu, just buy any tub of vegan protein powder and drink some. Same goes for supplements, take none.
   
   Why?: If you just make sure you're eating a measurable amount of "protein foods" (dairy, animal products, soy, protein powder) you will NOT miss the gains because of bad diet. You are picking up a new habit, don't pick up three or four packaged habits at once. Conditioning on this having any point, you'll be lifting for years. You can go back and re-analyze food if you want once lifting is sedimented in your lifestyle. Too many people fail because they try to "Do Fitness". Instead, just lift weights same as you would start going to a dance class. You'll get plenty of benefits. I'll make a post about more serious food optimization for when you're there.
    
7. Warm-up like this: For each exercise, if do two sets of that exact exercise, with less load/repts than you'll do in your "Work Sets" (which means non-warmup sets). This will both physiologically get your tissues ready, and gives you a chance to practice your technique.

What lifts to do?

A program I've prescribed often:

Here is a very basic sample program. I heavily encourage to at least go through Program Design 101 below, which will teach you how to substitute exercises if you need to.

There are two different routines, labeled "Day A", "Day B". Do day A. Rest at least 48 and at most 96 hours then do day B, rinse and repeat. You can do them on the same day of the week always, or not, does not matter.

NOTE: WHAT IS A SUPERSET? The entire program is "supersets". In fitness this is a loaded term, and the specific type we're using is "Antagonistic Supersets". Fancy sounding as it is, it's simply a way to cram more productive lifting per time unit. Take the first part of Day A: "DB Shoulder Squat" superset with "DB Lateral Raise". This means you do a set of the squats, rest only until you're no longer out of breath, like 30s to a minute, then do a set of lateral raise, rest 30-60s, go back to squats. Repeat until done the correct number of sets.

Workout A:

• 3 Sets of Squats with DB on shoulder , superset with DB Lateral Raise
• 3 Sets of Barbell Bench Press, superset with Barbell bent over rows.
• 3 Sets of Barbell Overhead Press, superset with pull ups (any grip, assistance form machine or band if needed)

Workout B:

• 3 Sets of Barbell Squats, superset with DB Lateral Raise
• 3 Sets of DB Bench press, superset with DB Bent over rows (both arms at once)
• 3 sets of DB Overhead Press, supersets with Chin ups (assistance if needed)

Program Design 101

This is just a template with which to make programs. It's the template I used to make the program above. The template for any day of this workout is

Workout _:

• 3 Sets [Legs] , superset with [Side delts]
• 3 Sets of [H Push], superset with [H Pull]
• 3 Sets of [V push], superset with [V pull]

You just fill in the brackets with exercises from the following lists:

• Legs: Barbell Squats, Leg Press, DB on Shoulder Squats, Front Squats, Goblet Squats, Bulgarian Split Squats, Reverse Lunges, Normal lunges, Walking Lunges
• Side Delts: DB Lateral raise, Cable Lateral Raise, DB Upright Rows, Side Delt machine, Leaning chair DB Lateral raise
• H(orizontal) Push: Bench press with dumbells or barbell, incline or flat. Push ups, Chest press machine.
• H(orizontal) Pull: Bent over rows with barbell or barbell, Inverted rows, Chest supported rows on a variety of machines, cable rows.
• V(ertical) Push: Overhead press with barbell or dumbell.
• V(ertical) Pull: Pull ups or chin ups assisted by band or machine if needed. Lat pulldown (cable) machine, DB pull overs, Lat Prayers.

How to pick from within a category? You need to have the equipment, and not hate the exercise, and if you can don't use the same exercise multiple times per week.

Final thoughts/ Random

• Why SUPERSETS? You probably want a couple minutes of rest between two sets of the same exercise, for those specific muscles to recover and be able to push again. Supersets are just a way for one muscle group to rest while the other does the work. If you just did squats back to back then lateral raises back to back, you'd either need to rest more between sets, taking longer, or perform worse in each set.
  Also shese are not all technically antagonistic supersets, since antagonistic would mean muscles that perform opposite functions get trained together. The legs + side delts combo is a "non-overlapping superset".
• Machines vs Dumbells vs Barbells: There is no broad difference. Barbells are better because you can load them heavier and in a more fine grained manner. Machines are better because with ingenius design, they allow you to train patterns that would be very hard to replicate with free weights, and while fatiguing you less. Dumbells are extremelly convenient, allow for single arm training, and for exercises in which an empty barbell is too much. I believe a beginner using good reasoning could get great results with just one of the three, or whatever mix they prefer. For each muscle group you want to train, look up "best way to train _ with DBS/Barbells/Machine" and the exercises that come up probably will be very good. 

Who is the author? I'm 24, have been lifting seriously for 3 years, and have not missed a week since I started. I've lost around 70lbs of fat and gained 15lbs of muscle in the meantime. I have spent an unreasonable about of time reading and watching things related to everything adjacent to exercise. From old soviet textbooks, modern textbooks, the vast knowledge dump from the 2000s blog era, to the modern youtube landscape where PHD's with professional bodybuilder level physique will talk your ear off about lifting. I don't follow much of my own advice. I lift 7 times per week for around 1.5 hours a day, and optimize my food and sleep for maximal gains. I train each muscle group approx 3 times per week, eat 160g of protein per day, and wake up without an alarm (can you tell fitness is my only hobby?), take 5g of creatine religiously. I have done paid personal training in the past, but it's no longer worth the money. 

Is something in this article not quite right? comment and I'll try to explain why I said it in more detail. I don't know if there is a market for people to hear my more complex advice for more advanced trainees, since I'm guessing lesswrongers who get into lifting quickly consume a lot of knowledge on it and don't need my summaries. This was inspired by an Anonymous friend who said he'd be interested, so I quickly wrote it up, since I believe nothing I am saying here is controversial advice.

Comment if you need anything answered, or DM me (does lesswrong do DMs?)