How do i gain weight with a fast metabolism? pls help?

ok well im 14 and im 5'7-5'8 and im skinny all ways have been people tell me you'll only gain weight when ur grown up but i cant wait that long i weigh about 120 lbs 54 kg's so if you know anything just tell me none of that junkfood crap i am an athlete and im going into freshmen year now so i need to be healthy and ill do weights and muscle building if you guys suggest but remember im a young teen so i cant use heavy weights.