There are 2 different ways to play it, depending on how high you want to play the riff.
It is in the Key of C, if that is any help. Also here are the notes, so you can play it 
on a piano if you want  C G C G G A E B x3 C A C A F C G
1. Pretty High
e-------------------|
a-------------------|
d-10----------------|
g---12--12-12--9----|
b-----13-----10-----|
e---------------10--|
Repeat x3
e-----------------------------|
a-----------------------------|
d-10----------10--------------|
g----------------------10--12-|
b-------10-----------10--13---|
e-----------------------------|
And that is it.

2. Low
e-8-------------|
a--10--10-10-7--|
d----10-----7---|
g-------------9-|
b---------------|
e---------------|
Repeat x3
e-8---------8------------|
b-----------------8---10-|
d------7---------7--10---|
g------------------------|
b------------------------|
e------------------------|

If you find the second riff difficult you could put a capo on the 7th fret.