First go to Settings>iCloud and sign in with your iCloud ID and password. Then go to Settings>iCloud>Storage & Backup and scroll to the very bottom under Back Up Now to confirm that you have a recent backup.
Assuming you have a recent backup, to restore the backup, go to Settings>General>Reset, tap Erase All Content and Settings, then go through the setup screens on the phone and when given the option, choose Restore from iCloud Backup and follow the prompts.
Be sure the phone is connected to wifi and your charger while it is restoring the backup. It will restore your intial data and settings, then restart and begin downloading your camera roll photos/videos and your apps and other purchased media.