@ Nayana
To reduce your stomach size, you need to have a combination of both aerobic and strengthening exercises. Abdomen is the natural reservoir for excess fat and because of this many people have protruded belly, though they may not look obese. So the prime aim should be to reduce the stored fat content of the body, which is possible by doing aerobic exercises. It is better if you start low intensity aerobic exercises and gradually progress to high intensity. Along with cardio, strengthening is also important as this will toned your abs and give them a good shape. If possible go for some recreational activities like outdoor sports, dance or some adventures like hiking etc. Along with this keep a check on your diet.