Fitness forums mostly died and Reddit inherited them. What is left is genuinely valuable and genuinely uneven, and knowing which communities are which saves a lot of wasted reading.
Start here
r/Fitness is the big one, over 17 million members. The discussion is too large to be good, but the wiki is outstanding and free: structured beginner routines, a proper explanation of progressive overload, and a nutrition section that does not try to sell you anything. If you read one thing on this page, read that wiki.
r/bodyweightfitness is the best place on the internet for training without equipment. Its Recommended Routine is a genuinely well-constructed program that thousands of people have refined, and it costs nothing.
Where experienced people actually are
r/weightroom is the serious lifting sub. Higher training age, better signal, and considerably less patience for beginners, which is a feature rather than a flaw.
r/naturalbodybuilding for hypertrophy discussion without the pharmacology, which makes the advice actually applicable to most people reading it.
r/AdvancedFitness is small and study-focused. This is the one place on Reddit where citing a paper is normal rather than showing off.
r/powerlifting if the goal is a total rather than a physique. Programming discussion here is a different sport from general fitness advice and it is worth knowing which you are reading.
By sport and by situation
r/running for everything from first 5K to marathon pacing. Its weekly threads are more useful than the front page.
r/xxfitness is women-focused and consistently one of the better-moderated fitness communities on the site, with markedly less of the aggression that characterises the larger subs.
r/Fitness30Plus for training around a job, children and a body that recovers differently than it did. Underrated, because most fitness content implicitly assumes a 22-year-old with free evenings.
r/homegym for equipment decisions, which is a surprisingly technical subject where the community knowledge is far better than any review site.
r/loseit and r/gainit for the two directions of body composition, both heavier on peer support than on programming.
r/flexibility for mobility work that is not just someone telling you to stretch more, which as it happens does not do what people think: does stretching prevent injury.
How to actually use it
Read the wiki before posting. Nearly every question a beginner has is answered in r/Fitness’s, and asking it again gets a worse answer than reading it would have.
Search before asking. Ten years of archives, and the old threads are frequently better than what you would get today.
For form checks, film properly. Side on, roughly hip height, the whole set rather than one rep, and say what you think is wrong. A bad angle gets you bad advice confidently delivered.
Sort by top, all time, in the smaller subs. The best content in a specialist sub is usually years old.
Which ones to skip
Anything built around a single supplement, product or personality. The incentive structure makes them a marketing channel with a comment section, however genuine the individual members are.
The physique-comparison subs, if you are at all prone to that, which most people underestimate about themselves.
And any sub where the top comments are reliably jokes. Fine as entertainment, useless as a resource, and easy to mistake for the second thing while doing the first.
For programs specifically, see free workout plans worth following, and for the video equivalent of this page, the best fitness YouTube channels.