I am 15 years old and I want to lose belly fat without going on a diet or using pills can I?

Yes you can, you do a lot of exercise to burn off the calories and the more you eat above what your body is using the more weight you gain. That is why many people combine a diet with exercise in order to speed up weight lose.

No diet is fine it just means you'll have to do more exercise in order to burn more calories than you consume and thus lose weight.