How to flatten my belly?

Crunches, push ups. : Abdominal crunches, pushups, and roman sit ups will target the area. But aerobic exercise like running/jogging, swimming, biking, or sports like basketball, tennis, or racquetball are all more fun and will treat your entire body. Have to follow a strict healthy diet too. Otherwise it's all pointless.

Lose weight: For flatter abdomen eat healthy & get physically active. Calories burned > calories consumed. To tighten ab's: crunches, reverse crunches, exercise ball crunches, pelvic thrusts, lying leg raises, side bridge & “bicycling” exercise on back. Engage in regular aerobic exercise (i.e. running, swimming, Nordic track, bicycling, kickboxing, walking, cross country skiing, hiking, etc.)