Ringtones are a fun way to personalize your iPhone. GarageBand, on every Mac, lets you create ringtones from music in your iTunes library.
Create a Ringtone Project
Launch GarageBand, select "iPhone Ringtone" (in the left column) and then double-click "Example Ringtone." Name the new ringtone project as you like.
Next, GarageBand will open the new project with a single track named "Jingles." In the track is a music snippet named "Pastel Slide Medium." Click on the snippet and press the Delete key to remove it.
Choose a Song
Click the Media Browser button (in the lower right corner of the window) and then drag a song from your iTunes Music library into the "Jingles" track. A bar with the title of the song will appear in the track.
Most songs are too long to use as ringtones. GarageBand uses a yellow bar (above the track) to let you specify an 18-second piece of the song to use.
Drag the yellow bar over the portion of the song you want to use for the ringtone. If you like, you can drag the ends of the yellow bar to create a longer ringtone. While the default 18 seconds is a nice length, 30-45 seconds is typical and let's you include more of the song.
Ringtones loop. So, if you let a ringtone play long enough when your phone rings, it'll play again from the beginning.
Send Ringtone to iTunes
Select "Send Ringtone to iTunes" from the Share menu. GarageBand will convert and export the ringtone to the Ringtones library in iTunes. The next time you sync, the ringtone will be installed onto your iPhone for you to use.