Buy one from the iTunes Store and install it, or create one yourself and sync from iTunes.
https://discussions.apple.com/message/17777381 - making a ringtone
Getting ringtones folder to show in iTunes - https://discussions.apple.com/thread/2629494 - iTunes > Preferences. Under the General tab below Show, confirm Ringtones is selected
How to sync ringtones to your iPhone - https://discussions.apple.com/docs/DOC-3792