i second what Mr. X said. For me when I was your age. As soon as I quit playing sports (football, track, wrestling) I started drinking more soda and consequently gain a good amount of fat.
I would have to say above all else, soda pop, and fast food are by far the top reasons people get fat easily. Next to those is junk food (chips, little debbie treats and the like (ding dongs, twinkies, etc)). As far as dieting goes, if you simply just cut those things out completely. And eat anything BUT that stuff, it is a much easier transition rather than having a specific 'diet' you have to follow