I'd be careful on what I took at 15. If you insist on taking a preworkout, then make sure it's a clean product WITHOUT any stimulants. The industry's go to stimulant for its products is caffiene, and lots of it. Caffiene lowers bone density...not caffiene abuse, but its very presence lowers bone density. At your age, your bones are still growing and forming. You don't want to take anything that could potentially compromise your physical growth. So when you choose one, choose one thats very basic. You could just make your own preworkout consisting of:
You'd have the creatine for cell volumizing and the arginine for your vasodiolation (your pump). Glutamine is the MOST abundant amino acid found in muscle tissue, and in intense physical training is used up fast. So the muscles need more to keep from going catabolic. Which is why I take glutamine preworkout instead of postworkout...I think its best to already have some glutamine on the way before going and depriving your body of it.The maltodextrin is a slowly digesting carbohydrate. It'll maintain glycolysis during your workout, so you wont run out of energy so fast.
I hope you haven't already made up your mindand get a chance to read this. Caffiene=bad, lil dude.