**EDIT** Link to an example:
Step 1: Set Up a New Garageband Project
In the screenshot, the key is set to F minor. I find this a good key to play around in, but this can change depending on what you are singing. If you are an experienced musician and know about notes and such, feel free to change this, otherwise stick with F minor.
Step 2: Create Your Track
In the bottom left of our interface click the "+" button. Select real instrument and create. The Last picture shows us what we now have.
Step 3: Setting Up Autotune
Take the "Enhance Tuning" slider and drag it all the way to the right and click the "Limit to Key" checkbox.
You now have Autotune set up, but to make it sound even better, we are going to add some effects.
Step 4: Adding Effects
We are going to look at the bottom of this list now and click the "details" button to open our advanced menu. The picture shows the effects that I used, but I will have them listed below. Feel free to play with these.
IMPORTANT: For the Distortion and Chorus effects, click to the right where there is a pencil, and it will allow you to edit it.
Compressor: Vocal Compression Rap
Distortion: Bring the drive to about 4.0 db (keep the rest the same)
Chorus: Bring the Intensity down to about 1.0 db (keep the rest the same)
Feel free to save these options as an instrument.
Have fun with your new AutoTune Effect!