How do I play a guitar in the key of C?

I'm a beginner guitarist and I'm trying to learn how to play Don't Fear the Reaper. I started playing the main riff I found on the Internet, but it sounded off. I went back to the website I got it from (http://www.guitaretab.com/b/blue-oyster-cult/1923.html) and it turns out that I'm supposed to play in the Key of C. How do I do this? I'm pretty sure it involves tuning the guitar a certain way.