its a mix between healthy diet and plenty of sit ups. most people think that if they do a gazillion sit ups a day then they'll get a six pack. Sure, you'll get a six pack will it will be hidden underneath the layer of fat. Unfortunately, the stomach is where is all goes on for men and a very low fat diet is the only way.
You need to burn off more calories than you consume. This can be done by a low fat diet, and regular exercise, mainly cardio.
I lived off low fat yoghurt and fruit for breakfast, cottage cheese, salad and ryvita and cut out bread altogether, while going to the gym 3 times a week and lost 3 stone.
Situps by themselves won't get rid of your tummy bulge, I can do loads of situps and attend karate classes twice a week but I still have a large tummy, as I am not dieting.