What is the best diet for losing belly fat for a 15 year old guy?

Hi! I am a 15 year old guy who is relatively thin but have quite a bit of belly fat. I would like to lose it by the beginning of school (5 weeks). I work out (15 minutes of running (Good heart rate) + arm workouts) daily. I would really like to know what the best diet is for doing this. Thanks!
will the south beach diet work?

and I read that spot reduction is impossible so doing crunches doesnt help losing the belly fat, all it does is build up the muscle underneath