Eat a lot of healthy food. Get you a good combo of carbs and protein and work out regularly. Lifting is better since cardio makes you lose weight easier.
Or you could say screw it, laze around all day snaking and eat a lot before bed each night. Ice cream in particular. But I don't recommend that as it makes you rather pudgy. Plus you'll have no energy to do anything. But at least it'll give you a belly, if you're into that sort of thing.