Well for a flat stomach, you can do planks, sit ups, crunches, 6 inches, bridges, but also it matters what you eat.
Isable R hit it on the nose. Planks, crunches, and bridges are great. Focus on working out your entire core (abs, hips, lower back, and butt). Building a little muscle in these areas will help a long ways. Your body burns off more calories to maintain 1lb of muscle than it would to maintain 1lb of fat. If the exercises above get a little boring (or aren't challenging) switch it up a bit. There are plenty of variations of these exercises at ab-workouts-guide.com that you can put into your regular workout routine.
Also try HIIT (High Intensity Interval Training), these exercises give you the benefit of building and muscle and getting some cardio in the same session. The short spurts will boost your metbolism and keep it spiked well after your workout, burning more fat ;)
Good luck and God Bless!