Katch,
What's your current routine look like? A very general set/rep scheme for hypertrophy (enlargement of tissue as a result of an increase in size rather than the number of cells) would be something in the range of: Sets 4-8 with reps around 8. Again this is very general as most react differently to routines and time under tension is also another consideration, but lets take this one step at a time for now.
At 15 yrs old I'd recommend you spend your time learning as much as possible. You're already ahead of your peers by training. You'll be much further ahead if you learn to train smart and not just hard. Biggest mistake I remember from my teen years is always trying to go heavy instead of strict. Now I'm spending too much time breaking old harmful habits.
You'll naturally get bigger in the next few years so be patient and smart.
In Health,
DM