Intro riff 1: 
A|-----2-2-3 
E|---3-3---- < after that riff play G major (listen to song for the rythem, repeat once) 
 
Intro riff 2: 
A|---2-2-3-3-2-- < after this riff play D major, listen for rythem 
 
Intro riff 3: 
A|---0-0-2-2-0 < after this riff play G to the same rythem 
 
Then play: 
 
G 
By the rivers of babylon 
G 
Where he sat down 
G            D 
And there he went 
D                  G 
When he remembered Zion 
G 
Alpha the wicked, carrry us away 
G         C                 G 
captivity requirs from us a song 
G                                      D       G 
how can we sing kind alpha's song in a strange land 
G                       D 
so let the words of our mouth 
D           G              D 
and the meditations of our heart 
D        G           D 
be exceptable in thy sight (strum D and then pause) 
     G 
over I (do not play anything at "over" and resume playing at "I")