How can i lose weight quickly and healthily???

im 14 and recently ive put on a few pounds. i now hate how i look and feel fat and frumpy!!! i want to lose weight but i want to do it as quickly and healthily as possible. i do lots of excercise and i am willing to do more. in the summer i like to run some nights after school but at the moment it is not light when i get back from school!! please help!!