a useful website when planning your Berlin trip is :
You can find the routes and fares for the Berlin Transport at :
The following address can be handy :
You can go to Postdam with the S7. I don´t know where it is your apartment located, but you can get the S7 at the Zoo station, Friedrichstrasse, Alex´... Quite easy. Once in Postdam, the bus 606 leaves from the train station to the Neues Palais (in Sans Souci), I don´t how often. There are also trams that go into the town. The official website for Postdam is http://www.potsdam.de/
I hope you have a great trip.
Kind regards, Covadonga Bilbao - Spain