They say that we store energy in our bellies, and the best way to get rid of that stored energy is by burning calories, not with crunches or sit ups. Aerobic exersize like biking or running is usually the best way to burn this stored energy fast, and of course to eat well and cut out the junk food.
That belly fat seems to appear over night. The best way is to start with a good diet. Do not add more to the problem that already exist. Start doing some belly crunches and some sit ups. Jogging will also help with this as well as keeping that cardiovascular system up to par.
Spot treatment of body fat is a myth. You cannot target belly fat. The human body loses fat everywhere or nowhere. Not in specific areas. As cliched as it sounds, limiting carbs and upping your protein intake help a lot. As fat as exercise, circuit training with weights (also known as Super Sets) are excellent for fat loss. I'm less a fan of cardio but if you must do it High Intensity Interval Training (HIIT) is the way to go. That means running extremely fast and hard for a minute and then slowing to a crawl. Thing about the way soccer players run on the field. Start-stop-start-stop...it's all in bursts. That's what HIIT is. Ever seen a soccer player without a 6-pack? I doubt it!
Years ago my sister would wrap her midsection, belly area, in Saran wrap, tightly, and exercise. It made her sweat and she lost weight, sometimes she would wear it all day and do her housework-- you can try that