George is essentially correct.
The only, and I mean ONLY, concern with using creatine at a young age is that there's a possibility it will down-regulate your body's ability to create its own creatine. It's the same sort of concept as steroid abuse causing your body to stop producing its own testosterone.
Whether or not this actually happens with teens and creatine is unclear right now. In adults, the body down-regulates its own creatine production while you're taking the supplement but then goes back to normal soon after you stop taking it.
It's up to you if you want to use it, but personally, I'd go for it if you have the money. Just remember to get a pure monohydrate (no sugar and other crap) and use a conservative dose (3-5g/day).
I wouldn't waste any money on a weight gain powder, I'd rather have a single chicken breast than a tub of that crap though.