Diet. HEALTHY diet.
1. Don't smother food (not too much ranch on salads, just BARELY there, not too much salt, etc etc).
2. Stick with fruits for sugars, but don't drink fruit JUICE that has TOO MUCH sugar added in.
Eat less and less progressively -- your stomach will stop getting stretched out from the HUGE ammounts of food you eat. Table manners also teaches you to have only one mouthful of food in your mouth -- don't shovel everything in. Helps you slim down. You get full faster and you eat slower -- enjoy your meals.
One BIG meal a day is also reccomended. Then spend the rest of the day being active! Walk EVERYWHERE, don't use the remote, things like that. Just do what you can to keep moving.
Also, to tone up that stomach, flex in place. Tense up your abs even while you're just sitting down. It's basically the same property as sit-ups or crunches.