Bodyweight chin-ups

Hello. I'm 16 year old male and just recently I started thinking about some bodyweight methods. I'm pretty skinny and I would like to improve that aspect of my physical health.
Especially, I'm not satisfied with my back muscles.
You see, I have talked about this with some of my friends and some of them recommend me to subscribe into some gym and start working with professional trainer.
But some of them told me that it could be pretty expensive and that probably the best thing for me at this moment would be some home training. What should I do? I don’t know anything about home training.

Hello. I'm practicing home training about ten years now and I can tell you that it really helped me a lot overcome my problems with being thin and skinny. Now, I'm very satisfied with the way I look and will recommend everyone to do this. You said that you are not so satisfied with your back muscles. That is very big group of muscles and the biggest muscles are big back muscle and trapezoid muscle.
The best workout for them is chin-up. Bodyweight chin-ups can be done very easily in your home and they are not dangerous in any way because you’re only lifting your own weight. Start with few repetitions because it may be hard in the beginning but increase the number of lifts with time. The traditional bodyweight exercises, beside chin ups, are knee-bends (squats), trunk curls, push-ups and bar-dips.