Want to decrease big belly, my whole body is still thin except belly.

Spot fat loss does: not occur naturally. By losing weight in general some will probably be abdominal fat. But, we don't get to choose what area of body fat is lost from. Eat healthy & be physically active. Calories burned must > those consumed. Tighten ab's: crunches, reverse crunches, exercise ball crunches, pelvic thrusts, lying leg raises, side bridge & “bicycling” exercise on back. Do regular aerobic exercise.