Baseball Training Throwing Drills

Im a 15 y/o pitcher/shortstop.
I was a sophmore this year. I made 1st team all-league. I had the control to get by but i was only throwing around high 70s and once in a while i would touch 81. The better hitters were hitting me but the rest i would shut down.

Im looking for a site or some drills that will get me throwing much harder for next season? I can train all year for it because baseball is the only sport i play and i have access to a gym with some mounds.

Thanks.
I also have a change-up that was my strikeout pitch. I throw a curveball once in a while too, not much though because my coach said because im 15 if i throw it a lot it would but a lot of strain on my elbow.