If you wanna go the free route audacity is a pretty powerful and straightforward DAW (Digital Audio Workstation). The interface is clunky and it can take some work to get it to actually work but I’ve had some ok results with it. I’ve heard great things about Reaper as well, i’m not a fan but a lot of people I know like it. And its sort of free.
As long as your DAW supports the vst format you can go ahead and grab G-Snap which is a fairly good auto tune program. It has its glitches but you’re not going to get the super polished auto tune for free. You can also try Kerovee if it still exists. I had a lot of fun with that one and its interface is a bit less taxing on your cpu. It does a better job if you’re going for the t-pain effect.
You can pick most of what I just talked about over at vstplanet. Its a website with a large host of vst effects, instruments and DAW programs. The best part is its all free.
If you can drum up around $100 usd you can grab a “starter” kit from most of your big name companies. Avid has a package thats an interface and a lite version of pro tools for around that still. I love pro tools but it can be a pain if you don’t know much about engineering. A lot of people who aren’t engineers like Ableton and Fruity Loops. If you go this route I’d recommend grabbing Antere’s Auto tune or Melodyne. Both are wonderfully powerful programs that let you get the auto tune effect.
I hope this helps and if you have any questions feel free to pm me.