There are loads of tutorials for Audacity online... but most of them I see are for PCs. Most of the tutorials say you need some plugin for Audacity. But I tried several plugins for Audacity and Audacity isn't able to run it.
Edit: Nevermind. I found it. Thanks \o/
Audacity is a free, cross-platform sound editing tool. Apparently it is possible to auto-tune in it. You have to jump through some hoops, but there are tutorials online.