This screencast tutorial gives you an overview of how to use GarageBand ’09 to record an audio essay and how to export the file as an mp3.
GarageBand comes free with all Macs as part of the iLife suite, which also includes iPhoto and iMovie. The version I’m demonstrating in the tutorial is ’09, which is very similar to ’11. If your GarageBand looks different from mine, you might have an older version. You can find plenty of tutorials on how to use it by searching Google, but the basic functions will be similar.