I want to make a simple vocal-based sound effect on OS X.
What software can I use to do this? Is it easy to do with Garage Band? Is there a specific tutorial I should look at?
If it is a vocoder you are looking for, the only one I know of for mac is a module inside of Propellerhead Software's Reason (a great piece of software, but not a bargain - $499). If you go this route, you can record your vocoder track and import it into GarageBand (or Logic). http://www.propellerheads.se/products/reason/index.cfm
This may not be what you were asking, but you can open a terminal and type
say -o out.aiff "Hello World"
to write a sound file with text-to-speech output in it.