Probably the one thing that everyone wants is a flat stomach and despite what you might have read elsewhere, there is only way to do it and that is to burn fat. Now burning stomach fat cant be done directly, there is no single exercise or diet that will result in a spot fat reduction from any part of the body. Instead you have to lose weight consistently over a period of time and once adequate amounts of fat have been burned from other areas, then it will start to drop around your stomach.
The first parts of the body that loses fat first varies from men to women and from person to person but the stomach is usually the last. Burning stomach fat will be hard work and its something that you must really want to do in order to have the kind of commitment you are going to need.
Now I am not saying sit ups and the like are useless, they tighten your stomach and make it look a lot smaller. Do that for long enough and if you are a guy the six pack will grow so that when the day finally comes that you have burnt off enough stomach fat then it will start to show.
I could go on, but to cut a long story short, get into a weight loss/fat loss program and make the concerted effort and do not believe anything or anyone that says otherwise.