This is the main riff, it is played through the whole song: just repeat it.
G |: ----------------|
D |: ----------------|
A |: 3-------6---1---|
E |: ----4-----------|
Or you can play it like this:
G |: ----------------|
D |: ----------------|
A |: 3-------6---5---|
E |: ----4-----------|
And really that's it. Please tell me if it needs to be corrected.