As long as you had iCloud Backup enabled, all you need to do is restore from your iCloud backup. I don't know how to do this directly, because I don't backup to iCloud. However, I'm going to outline how I would attempt to restore from an iCloud backup.
Originally Posted by deanh8
1. Backup (if you have anything on your phone currently, just in case you need to get back to it) by plugging your iPhone into your computer letting iTunes do its thing
2. Unplug your iPhone from your computer--it'd probably be a good idea to plug it into a power source
3. Open 'Settings'
4. Scroll alllll the way to the bottom to 'Reset'
5. Open 'Reset,' select 'Erase all content and settings'
6. Once everything is erased and your iPhone reboots, you should be at the setup screen
7. When you arrive at the 'Set up iPhone' screen, select 'Restore from iCloud backup'
8. Follow the instructions, and that should work, provided you had a backup
Addendum: to elaborate on #8, start at about 1:50 on