If it is central bulge and stick out during sit ups , consult surgeon. But if it is generalised bulge, you need to combine exercise with diet modifications. Increase your protein intake significantly ( 1 gm/ Kg body weight) and reduce carb intake significantly. Avoid fatty food, increase intake of fruits, salads, sprouts. Drink plenty of water. In exercise , only sit ups will not help you. You must combine aerobic exercise ( swimming, cycling , running, brisk wallking, dancing) with weight training. Becasue unless and until you lose your body fat, you can t reduce your belly. Sit ups should be combined with these exercises.