Ringtones transfers any ringtones you create to iTunes, but you must sync your iPhone before they become available for use.To sync ringtones, click on your iPhone's name in the top right hand corner of the iTunes window (iTunes 11) OR click on your iPhones's name in the menu to the left of the main iTunes window, under 'Devices' (iTunes 10 and below).
Then, select the "Tones" tab from the menu near the top of the iTunes window.
Check the box next to "Sync Tones" then hit "Apply". Make sure you have selected to sync the tones you are after.
Then hit "Sync" and leave your iPhone connected until the sync has completed.
You can set the ringtone through your iPhone's settings.