If I were you I'd stick to calisthenics, and avoid heavy weights. 15 is rather young to be putting high stresses on your bones and ligaments. I would build a decent strength base first. So do tons of push ups (narrow grip, wide, inclined etc.), Chin ups (close grip, wide, etc.) dips, crunches, leg raises, hyper extensions, lunges and stuff like that. Then in a couple of years, you can start thinking about lifting heavy.