flat stomach…?

can u get a flat stomach just by going on a diet and losing weight or do u have to exercise, too? if u do have to exercise, what exercises do u need to do? i have been on a diet since january and i’ve lost 20 lbs. so far, but for some reason i still have "that stomach". how do u get rid of it..? does this mean i need to lose more weight or just tighten up or something..? i want to get rid of it b/c i feel very self-conscius wearing a shirt and hiding my stomach and all. please help & thank you!