I think he's looking for a stimulant that gives you energy before a workout. Seeing that you are 15, you should have enough energy as it is. If you don't, then make sure your diet is in check. But, a half of a caffeine pill (100mg, as much as is in coffee) sometimes works for me if I'm feeling particularly lethargic.
But you know what, just eat big, lift big, and grow. You won't benefit at your age from taking stimulants. It will probably only have adverse effects.