Hi,
I think there is the S-bhan that runs directly from the airport into Berlin city centre stopping off at places where you can connect onto the U-bahn..
The links can be found here with what appears to be good information on Berlin;
http://www.btm.de/english/anreise/e_an_flug.html
(Scroll down for Schonefeld S bahn info)
Hope this helps
Regards
TBS