Found this on AC, for use with Garageband as well:
These days, it seems like everything they play on the radio has some sort of autotuned singing. Autotune is the effect made famous today by artists like T-Pain and Kanye West, the latter releasing a whole album (808s & Heartbreak) using the technique. Some swear by it, some condemn
its usage, but regardless of your position, I guarantee you that you will have fun playing around with this effect.
There are various ways to get the autotune effect on GarageBand; however, I will show you how to do it without downloading and installing any plug-ins, completely hassle-free. Here are the steps to follow:
1. Open GarageBand, go to the Track menu and create a "New Basic Track."
2. At the bottom left of the window, click on the icon that has scissors. This is the Track Editor.
3. Slide the Enhance Tuning to 99 (max) and check the box next to "Limit to Key."
4. Now that you got the basic autotune effect, you can play with sound effects by clicking the "i" icon to the right of the master volume control. Click "Vocals" and try out effects that you like.
Remember, this may not be the best quality autotune effect out there, but it is definitely the easiest and cheapest way to get the expensive effect (buying it can cost around $100). Mess around in the program, tweak things as you fit to get the sound you desire, and record - whether it be for fun or for your musical work. Now you can mimick T-Pain with the basic GarageBand software.