What workout should i do to get a flat stomach?

You don't need to go on diet! Here is my advice! It is enough to make 24 sit-ups daily to have great abs. But, this way you will only get abs! The other important thing is to get rid of your belly if there is any! :o)
To do that, you have to increase intensity of leg training! You can run or do anything that applies leg and buttock muscles training! The abdominal muscle group can be compared with a small car which has low fuel consumption. On the other side, buttock and thigh muscles have high fuel consumption like a big truck. The goal is to assure high energy consumption on training in order to force the body to waste fat stocks. So, what you will do? Work out on your legs, of course!

See more of informations about this topic at:
How To Get Abs