I wish people wouldn't suggest a factory reset quite so blithely. It can be a royal pain to recover from, and it should be the last resort, not the first thing to try.
Crystal, did you recently install any new apps? If so, you might try uninstalling them, reboot, and see what happens. Otherwise, try a couple of other things first:
1. Go to Settings / Display / LED. Uncheck all the options, then reboot, and see if it goes away. (I'm assuming you're not charging the phone--the red light is supposed to be on then.) If that works, then re-check the appropriate options.
2. If that doesn't work, try clearing the phone's cache. Boot into recovery (Turn the phone OFF, and wait for it to shut down completely. Press and hold, at the same time, the volume UP, home and power keys. When you see the Samsung logo, release all three buttons. If the phone boots normally, shut it down and try again.) Use the volume buttons to select the option to clear cache, then the power button to execute. Reboot the phone.
3. If that doesn't fix it, try going to the Application Manager, and tap Menu / Reset app preferences.
If none of that works, then a factory reset might be the next step. You'll want to back up as much of your data as possible first. Check out the Helium app in the playstore, or use Kies.