One thing to remember, you can't "spot lose" body fat. Sometimes people tend to focus on working out a specific body part (like the abs) thinking that will reduce their waistline. The only thing that will reduce your flab is lowering your body fat percentage. This is primarily achieved through reducing caloric intake, and doing cardio exercise, whether that's jogging, using a bike, whatever. Building muscle can help you burn more calories long term as well, as others have pointed out.