I'm still in my teens, I eat fast food alot, yet I'm skinnier than my other friends, I guess it's about how high your metabolism is. Limit the sugars and start eating lots of fruit, preferably apple, because apples give you negative calories, yet it gives you the proper nutrients you need to stay healthy.
If you want to be "leaner", try doing pushups against the wall when you're in the shower or something if you don't have the time to work out. 50 each hand.