What excersices flatten your stomach? Will eating less help me to lose weight faster?

Yes. Exercise and dietary restriction of calories always helps in a diet.
Yes - watch diet. 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.)