there are a bunch of supplements that do a bunch of different things.
at 15 years old, i would stick to mainly protein powder, flax seed oil, fish oil, omega tabs/powder, glucosmine and chondrotin, glutamine, multi vitamin, BCAA's (Branch Chain Amino Acids).
but really, all you need is a good protein powder.
ON whey is a good place to start.
this is my stack
true mass syntha 6 animal flex
size on (soon)
superpump 250 (even though i hate NO)
epistane