It's beating a dead horse here, but if you're that young, EAT EVERYTHING! Want a bag of Twizzlers? Eat 'em. Want a gigantic wet burrito? Four burgers? Eat 'em. Keep eating; keep snacking. High fat, high sugar... whatever. At that age, seriously you can stack some serious weight on in a short period of time.
When I started, I was 5'8" 139lbs. I started drinking this shake called Muscle Blast 2000. It was a 2000 calorie shake that I'd down every day in addition to hitting the chow hall and eating omlettes, ham, bacon, hard-boiled eggs, shjt on a shingle, and anything else I wanted, INCLUDING CANDY. I put on 40 pounds in four months without moving my belt notch; I was 19 years old.
There is no better time than now to put that weight on, and enjoy gluttony.