The only way to ultimately change your body composition is through proper eating and nutrition. No amount of sit-ups or core exercises will spot-reduce that "muffin" or spare tire around the waistline - or anywhere else on the body. While daily caloric intake is very important, what is even more crucial is where we get our calories- from carbohydrates,fats, fiber and protein. So many people think that carbs are the enemy, but carbohydrates are the fuel that drives our engine. We need to ingest the right types of carbs such as fruits, vegetables, legumes, whole grains, brown rice etc and stay away from too much bread, processed cereals and sugars. Another mistake some people tend to make is to reward themselves for beginning an exercise program -they soon find out that they're sabotaging their own efforts to lose weight and achieve lean muscle definition. The other mistake is when people starve themselves, so-to-speak, by not eating enough. The body knows when daily requirements to function aren't met and will conserve energy stores as a result.
If you're having trouble getting on top of your nutritional needs, don't hesitate to seek help from a qualified dietitian or nutritional counselor.