I want to lose weight but I can't stay motivated?

I've wanted to drop 15 lbs for a while and i can't seem to stay motivated enough. I'm 5'3 16 and 130 lbs and I'd like to be 110-115. I want to be thin for junior year so how many calories should I eat a day, and I can do cardio I have an exercise bike and a gazelle trainer how many minutes a day to see results. Also how can I stay motivated to stick with my diet and how long will it take? is it reasonable to want to lose the weight by the end of summer (september 7th) if I start this week?