Open a GarageBand session.
Select "Share" in the GarageBand menu, then select "Send Song to iTunes."
Fill in the fields for iTunes Playlist, Artist Name, Composer Name and Album Name in the pop-up box.
Check the "Compress" box in the pop-up box. Next, select your desired compression setting under the "Compress Using" function. Then select desired audio settings.
Click "Share."
Open iTunes on your computer once sharing has completed (sharing can last from a few seconds to a few minutes, depending on the file size). Find the song in the playlist, artist name, composer name or album name you designated.