I'm 21 years old and I found that I got the most flat/toned stomach when I began weight training. (Incorporated with ab workouts after). When you build muscle your body will burn more calories at rest than it would with less muscle mass. Incorporate cardio too, about 20 minutes. I also eat a diet with a lot of whole grains, fruits veggies, and no junk food like chips or anything like that.
Have you tried anything besides ab workouts? What about healthy eating and other exercise?
At 14, your body isn't fully developed, so you'll need to get your parents to help you.