How to lose at least 5lbs a week?

im 12 yrs old nd to me im overweight i weigh 137 and im scared i made it on the school drill team and uniform fitting is 3 weeks i wanna lose weight to look pretty in the costume. any ideas or exercises or diets so i can lose at least 5lbs a week my goal is to weigh 115 or 110 by the end of julyy HELP!!