Are you on the newest firmware? if you are in low light try night mode, any camera will be grainy in bad conditions. Try clearing the camera app cache and reset the settings. Try clearing the phone cache in the recovery menu. If this all fails and your photos are still more grainy than others online, contact Samsung and see what they say.
Did clearing the app cache help? Also, try using natural tone for the selfies as another option, and check to see if beauty mode is on, sometimes it does strange things to the photos.
