Hey,
well my 2 pence to this thread will have to be that MIDI is not an audio format, it is (amongst other things) a language electronic music devices use to communicate and/or store note information and other music related information.
For example, an mp3 file may for example contain an audio recording of a guitar solo. This recording is actually a very long list of amplitude values which, when reconstructed in the correct order and played at the correct speed sounds like a guitar solo.
A MIDI files representation of the same guitar solo would contain absolutely no audio. Instead a MIDI file would contain a description (still digitally written in 1's and 0's) of every single note the guitar player played, how hard he/she hit the note, when they hit the note, for how long they hit the note, how the note was bent... possibly over 100 different pieces of information about that note. You would then have to use a MIDI sequencer and a MIDI sound module to play back that data in audio form.
Now I would imagine that although you are not accutely aware of it, your garageband song contains both audio data and MIDI data, which you have then mixed together in audio form and 'bounced' into an AAC file. You can convert this pretty easily using iTunes. First go to preferences, then accross to the advanced tab. Within the advanced tab go down to 'importing' which is another tab within this tab. Click on the 'import using' dropdown menu and click mp3.
To change the quality of the mp3 file (i.e. the bitrate) you can hit settings and go across to custom. Here you have control over how the mp3 file is made. The higher the bitrate the better the quality. I suggest you would not be able to tell a great deal of difference over about 192kbits... obviously higher quality = bigger files. If you are playing back over the web this isn't a good thing. If you want to save space by making your file mono you can do that. I wouldn't touch the sample rate. I am guessing you have recorded at 44100 samples a second. Increasing this to 48000 will not improve the audio quality of your recording at all, it will take up more space however and could potentially be less compatible with other computers soundcards.
Now, go back to your iTunes libary, find your song and right click it (ctrl-click). Hit the convert selection to mp3 option. iTunes will now create a new file in the mp3 format, and place it next to your old file within your libary.
Hope this helps,
Danny