How do i stick tom a diet? (15 years old)?

EAT MORE. You don't need to diet, but if you would like to burn stomach fat eat around 1500-1600 calories until you do, but you must do cardio every day. Also for breakfast get rid of the honey bunches of oats, there are so much better options, try eating oatmeal it is way better for you and get rid of the dessert or choose something like fruit over that.