PLEASE PLEASE help me?

okay i am 13. iam slightly over weight and need to loose about 15-20 lb. to be the weight i want. i have a month. any ideas? i am 5'1. i already eat SUPER-heathly with no sugar and no gluiten. i try to exercise, but never get as much as i'd like in. i usually bike because i have flat feet, and its painfull to run for a long time. please any ideas? thanx a ton