what to eat and what not to eat when losing weight?

I'm seventeen years old and I want to lose 30 pounds, but I don't really know how to change my diet. I've been the same weight now for five years. The biggest thing I need help with is changing what i eat, because i actually don't mind exercising. I've heard it's best to eat a big breakfast, a medium sized lunch and a small dinner. But I don't know what to eat at those times. What is the key to losing weight? I want to be at my goal weight by graduation, so I have plenty of time until June. Thanks in advance!