Most of the Bayern-Tickets, particularly those purchased from Bahn automats, come out of the machine with the date of validity printed on them and don't need to be validated. (They will also be too wide to fit in the cancelling machine.) Those that do need to be validated will have the words "Hier entwerten" printed on one end with arrows pointing to the direction of insertion into the blue cancelling machine that is located at the entrance to the station platform. You will, however, need to print the name of one of the people in your party on the blank line. That person must be with the ticket for it to be valid.
For Garmisch-Partenkirchen, take either S-Bahn, S1 or S8, whichever is leaving next, to the Tief (underground station) at the Hauptbahnhof. After coming up one level from the tracks, follow the signs saying "27-36" (tracks 27-36 in the Starnberger Bahnhof, a wing bahnhof attached to the north side of the Hauptbahnhof). If you find youself in the main shed of the Hauptbahnhof, follow the north wall out along track 26 until you find an opening leading to Starnberger.