The whole song is basically D's A's and Em7's when you play the D strums instead of
from down to up try it vice versa; you beat the first string at the bottom of the guitar so 
it goes and i hope you enjoy it....

D (xx0232@1)     A (x02220@1)  Em7
Yeah, yeah, yeah
D (xx0232@1)                    AMama (x02220@1)take this badge off of me
Em7
I can't use it anymore
D (xx0232@1)                           A (x02220@1)                                Em7
It's getting dark to dark to see, feel I'm knocking on heaven's door
[chorus]
D (xx0232@1)                                  AKnock, (x02220@1)Knock, Knocking On Heaven's Door
Em7
Knock Knock Knocking On Heaven's Door
D (xx0232@1)                                  AKnock, (x02220@1)Knock Knocking On Heaven's Door
Em7
Knock Knock Knocking On Heaven's Door
D (xx0232@1)                        AMama (x02220@1)put my guns on the ground
Em7
I can't shoot them anymore
D (xx0232@1)                             AThe (x02220@1)long black cloud is coming down
D (xx0232@1)               A (x02220@1)                 Em7
I feel like i'm Knocking On Heaven's Door
D (xx0232@1)                                  AKnock, (x02220@1)Knock, Knocking On Heaven's Door
Em7
Knock Knock Knocking On Heaven's Door (Repeat as the first one but let the Em7 last and 
and don't stop it)Hope you enjoyed it!