Eat less overall, with more fruits, veggies, etc.
That same advice always works.
You can eat some junk in moderation every once in a while, just as long you keep your overall caloric intake relatively low for quite a while. When buying stuff, always look at calories, fat, calories from fat, protein, and avoid stuff that has excess amounts for a given serving. Summer is a great time to lose weight; without school, you can go to the gym more, do more exercise, etc. Drink lots of water, and try drinking some cold water right after you wake up to kick up the metabolism a bit.
I also used to have a weight problem but I guess the difference was that I grew 3inches last spring/summer in addition to all my exercise, so I got a lot leaner.