To reach Wannsee take the North/South train line on Berlin's excellent public transport system. From the Wannsee station it is a short walk to the bus stop. A circular bus route runs around the area and drops off outside the Villa - the stop is 'Haus De Wannsee Konference.' The same bus will take you back to the train station, where you can catch the same train back to Berlin centre.
Wannsee is a worth while half-day trip from Berlin, and it is very easily accessible by S-Bahn line no. 7. You can get on this train in many stations in Tiergarten, Hauptbahnhof and Mitte, and the trip is considered within "zone B", so that if you have a day-pass or a 7-day-pass for zones A&B this covers the journey to Wannsee.
On the way I would recommend dedicating 15 minutes of your time to get off the train in the Grunewald station, go to Platform 17 ("Gleis 17") and visit the monument to the Jews transported to concentration camps and death camps from that very platform during the Nazi period. Then you can board the next S7-train to continue to Wannsee.