Exporting Your Song
If you want to work on your song later, you should save your project as an archive. Go to File > Save as Archive. This creates a file in the GarageBand folder that contains all of the loops in your song, as well as your Real and Software Instrument recordings.
To export your song as an audio file, go to the Share menu and choose Send Song to iTunes. iTunes will open and start playing your song. You can then burn it to a CD or move it to the desktop by clicking and dragging.