How can I lose weight (get a flat stomach...) quickly, yet still do it the healthy way?

So, I have always been pretty fat, then I got down to just kind of chubby (SUCCESS lol) but I'm on Varsity cheer now, and it just seems right to be fit, and have that flat stomach that every girl wants. We recently (last week) started running 3 miles every evening, and REAL practices are about to begin, however, I'm not sure if that is enough. I try to eat healthy, but its difficult to do sometimes when I'm stuck with whatever my mom brings home. School starts August 24th, and I'm not saying I need it done by then, but I do want to be well on my way...any help is appreciated. Thanks!!