Since I was like 12 I was blessed with the genetics of an 80 yo. Having muscle spasms, hernias, and all sorts of bizarre crap. The two times in my life I got into a workout regime things would get progressively better until months down the line, when I'd have to do surgery of some kind, likely because I 'worked out too much'. Doctors tell me to start lifting weights, but they don't seem to believe me and simply tell me I'm a male and young and shouldn't be as frail as I am. No tests in 20+ years have shown anything out of the ordinary, except for whatever problem I developed all of a sudden, so it's no wonder. I'm afraid that trainers in a gym wouldn't be any better. Got no idea where to start. Pool workouts with the elderly maybe?
You don't need to use weights to build muscle with bodyweight exercises like calisthenics. Your body can't tell weather the resistance is coming from external weights or your body. And you can start off at the beginning with the progressions, like push ups against a wall, rows in a standing position, bodyweight squats, etc. The hybrid calisthenics YouTube channel is a great resource there. That's where I started. Just stick to getting close to failure in the 6-30 rep range (even higher if you're just starting out), take adequate recovery time between workouts , and you will get stronger.
I second this, hybrid calisthenics has a good website and trains flexibility and movement (not only strength like others) which is probably what OP needs.
If anything is too hard for OP then the guy has easier versions of every single exercise. As long as OP follows the steps and advice for how many reps he should be able to do for each exercise before moving on to the harder versions then he should be fine.