So from this point on, you're done with Garage Band and iTunes. All that is required is to get the file online.

Open up a web browser, and go to mix2r.com. In the left hand navigation menu is an item called "upload audio". Click on it.

When the page loads, fill in some information -- notes for whomever visits the audio file are nice, perhaps indicating what kind of things you need help with or simply want your audience to know. Also, select an appropriate license and mark the file as having been produced in Garage Band.

Next, add the MP3 file. iTunes music is generally stored in your home directory under Music:iTunes:iTunes Music.

Once you've selected the file, and if applicable, the audience (groups) that you'd like to notify of the new file, click submit.

...and voila, you're done.