Sit-ups! I have one too on and off for several years. When I start doing a lot of situps it shrinks. Aim for about 30-40 a day. In the beginning you might only be able to do 5 or 10, maybe even less.
Also, losing weight is the best way to lose it.
What you see isn't really a big stomach, but something called the omentum. Its like a sac of fat that covers your whole ab area, but if your abs are strong, the belly won't look so round.