At 15, you need to get GOOD habits started. Go to a gymnastics gym. Why? Gymnastic will get you set onto a progressive strength program. The body of a gymnast is gained as a BY PRODUCT (meaning, it looks that way because of the activities they do, they don't try to get a nice looking body on purpose).
At 15, it'll be hard for you to dedicate yourself to a regimen or even just a diet without a good support group(friends) or activities to motivate you to do such. Gymnastics will not only make you look better and get stronger as well as flexible, but it will make you 2178640821376873465% more aware of what your body is doing and capable of.
As far as supplements go, the ONLY supplement I would look at it protein. Unless there's additional things your gym coach tells you to start adding to your diet.
Diet is key. Without it, you're gonna lose.