I did research on this so trust me.
Either decrease your calorie intake (assuming that you are a teenager I don't recomment this since you are growing) or increase it and burn the calories you increased. Let's say you eat 1500 cal. per day. You increase it to 2000. Then you have to burn 500 calories. Don't decrease it and never go down a 1000 cal diet. I consumed about 700 calories a day for two weeks and I got dizzy.
You should do cardio workouts which are running, dancing, swimming, etc. Whatever makes your heart rate faster. Cut out the bad food from your diet. By experience, dairy makes you fat. I cut out dairy from my diet completely and I've noticed differences. You shouldn't eliminate dairy because you do need your dairy intake; I just did it because I was intolerant.
And also from experience. I wanted to do an experiment to see if I look any different doing 20 min of crunches everyday. Nothing happened. You have to lose the fat first then do crunches. Crunches helps your abs get tone, not make you lose fat.