This is can be done very easily with a few simple steps.
First open GarageBand.
Step 1: Step Dos.
After you've opened garage band record any voice clip you want wether it be from the built in mic. or an actual recording device and select it.
Go to the bottom left corner and click on the little button with the scissors on it.
Step 2: Finish
After ou click that button a sub menu will pop up. Gram the slid knob under where it says "Automatic Tuning" to 100.
Check the "Limit to Key" box and your voice recording should be auto tuned!