How do lose all this puppy fat?

Sure, in fifth grade we were all somewhat chubby. But now in seventh grade I feel really huge! My clmates all lost their extra weight and all of them look great except for a few boys whose trademark is their obesity. Anyway, I hate my flabby gut and I really want to lose it! I'm about 5'3 (yes, short, I know). What should my weight be? I know I'm a bit overweight and I want to lose as much as possible by, I don't know... Christmas? Easter? Anything! Any advice will be appreciated!!