1. Get your phone and hook it up to iTunes.
2. Once connected, right-click on the device in the list and select 'Backup'. You might also want to do the rest while you're there too for safety such as 'Sync' and 'Transfer purchases'.
3. Restore the iPhone either through iTunes (hit the 'Restore' button when the device is connected)
4. When finished, you'll be asked to restore from backup or set up as a new phone. Pick the second option (backup) and select the backup. It should be the one from Step 2 (note the date/time).
It will only delete everything from the iPhone, not your iTunes library/backups there. Sync and backup first before you do anything.
Failing the above and assuming you follow the first recommendation, try resetting and restoring from iCloud.