There's hardly any parking space at the stadium. What you can do is park your car at park and ride Altglienicke, from where you take the S-Bahn to station Schöneweide. Here you can change to tram 60 or 67, which bring you to the Alte Försterei.
From the city centre of Berlin you best take the S-Bahn as well. Line S3 passes the Hauptbahnhof, the Alexanderplatz and the Ostbahnhof, and if you take it in direction of Erkner you get off at Köpenick. You're immediately in the heart of the Union district and the stadium is a 10 minute walk from the station.