How to data block an iPhone 3GS?

I'm a broke college student about to graduate and can't afford any fancy phone but need to check my email all the time. And there's Wi-Fi pretty much everywhere you go nowadays so why bother with buying a data plan. Anyway, I just want to figure out how to data block it so I don't have to keep calling AT&T back every month.

Go to settings->general->cellular-> and switch it off