14 years old, female, 5'7 and 128 pounds. How can I lose weight before high school?

So I am a 14 year old girl, 5ft 7in, and I weigh 128 pounds ): ! I really need to lose about 13 pounds before august 20th because school starts september 1st. I have always been thin but now I am looking pretty chubby. I know that 115 pounds might not be too healthy, but I am shooting for this number. Highschool is really important and I don't want it to be revolved around my weight like 8th grade was. I literally went from 114 pounds to 133 this year. That is almost 20 pounds of weight gain. Luckily I have already lost 6, but I really need help with food choices, calories, and exercise. If anyone could help me out that would be great and I will give them 10 points!