GarageBand allows you to create audio projects and save them in formats compatible with by other applications.
Save audio to disk
There are two ways to record the song to disk:
Transfer your file to iTunes: Once you transferred a song to iTunes, you can include it to a playlist, transfer it to your iPod or burn it on a CD.
As a AAC or MP3 file: To save the file directly on the hard disk of the computer, choose Share > Export song to disk, then choose the format (MP3 or AAC) and the recording quality. Finally, click Export and select the name and location for the file.
Note that: By default, iTunes converts imported music in AAC format which is the main compression format chosen by Apple for iTunes and iPod.
To transfer the song to iTunes:
Choose Share> Send Song to iTunes.
In the window that appears, enter the appropriate information in the text fields, check Compress, then choose the compression and audio settings.
Burn to CD
When you burn a project on CD, only the audio track will be used. If the project contains illustrations, URLs, or video, they are not written to the CD. If the project contains chapter markers, they will be used to define the tracks on the CD.
Insert a recordable CD into the optical drive of the computer.
Choose Share> Burn song on a CD.
To burn multiple songs, save them in a playlist under iTunes and then burn them to CD.