I don't know whether Garageband exports to mp3. You find out yourself. Try the online help for a start.
I can tell you how to convert an AIFF to mp3 though. Drag the AIFF into iTunes, then go to iTunes->Preferences->Advanced->Import and select the mp3 encoder with desired settings. Leave the preferences dialog, select the AIFF file and choose the menu Advanced->Convert selection to mp3 (the menu reflects the type you have chosen in the preferences, so you could also convert to AAC or Apple Lossless with this menu command).