How can a 13 year old girl lose weight?

I'm 13 I'm 5'5 and weigh 128lbs. My weight changes alot, mostly between 125-128 and I want to get down to about 118 or so. I mostly just lewd help with the last 6-10lbs. In the beginning of January I weighed about 140 so I went down a lot. Right now I'm going to eat around 1400-1500 calories and burn about 500 a day. I would run on the treadmill and ride a bike. Also I might start P90X Lean. So how long would it take me to lose the weight? I want to be 120 by my 8th grade graduation at the beginning of June. Also what are good diet plans that I could try? Any tips on how to lose weight fast? Should I exercise more? And what are good foods to eat as a late night snack? I get hungry at night.... Haha thanks(: