Nipples are like breasts, penises and all other body parts in that they all come in different shapes and sizes that make each of us unique. A flat nipple is not considered a medical problem and will not interfere with your sexual functioning.
Plastic surgery can change the shape of your nipple however there is no guarantee that they will not revert back. Also, surgery may alter the milk ducts that could prevent you from breast-feeding in the future and change some of the sensation in the nipple.
As with all matters related to appearance, TeenHealthFX encourages you to look beyond the imperfections you may see with your body. If you examine anything too closely and critically you are bound to find some flaw. However, it is these differences that make us unique. Try to accept your body for the way it was meant to be and resist the urges to conform to how you think it should be, which is often influenced unrealistically by the media.