First exercise you should be doing for good ab development are leg raises. These are easy to do, and develop good lower ab muscle. Second exercise are regular crunches, you don't want to do too many of these. You can do 2 sets of 20 reps for these to start it off. Then add an extra set every week. Third and final exercise would be decline crunches, for most teenagers, these work very well, and build strong hard abs.
When I talked about dedication earlier on in this article, I wasn't talking about training. Oh no no no. I was talking about dieting! What most teenagers don't know, is that dieting is one of the most important factors when it comes to bodybuilding. Bodybuilding isn't the only thing there is to build muscle. High protein diets are the way to go in this case, you want to keep your protein at 1g per pound of bodyweight, and the rest of calories you can fill up with carbs and healthy fats. Now you want to stay at about 2500-3000 calories to build muscle and stay at your current weight. Obviously, you don't want to gain too much fat knowing that you want a 6 pack for the summer.
This was just a little article to sum it up, good luck to you guys!
