Mastering in Garageband
First start by creating your song made up of many tracks in garageband. Mix the song so everything sounds good to you. Adjust levels for each track, adjust EQ for each track, adjust panning for each track, and standard effects like reverb and delay. (etc...etc..)
When you are done and the song sounds pretty damn good, export the song to itunes as an AIFF format which is a lossless file format unlike MP3...
next, create a single track garageband file, and import your AIFF file from itunes.
Now you can master this file in garageband!
Listen to the song on a couple sets of monitors/speakers from big fancy ones to little cheap boob box type... and also listen to your song in headphones. Doing this will reveal where you need to adjust EQ... and compression, etc...
Then if you really want a great soundm get the plugin called OZONE 4 (or maybe there is a newer version). This plugin makes your song sound amazing!!
Once you have done your critical listening on various speakers and headphones, you are ready to export again to itunes. Output as MP3 for ipods/iphone, etc... and as an AIFF if sending your file to a CD production facility. This is your MASTERED song!
Make great music! and share it on www.soundcloud.com!! Sound Cloud is the facebook for MUSIC!!!
Check out mine at http://soundcloud.com/svend007
Thanks for reading!