Originally Posted by
Zaphod
You could take the S-Bahn (suburban train) from Storkower Str to Beusselstr and change to the airport bus ("TXL") there.
If you want a more scenic route, you can also take the Subway (U4) to Alexanderplatz (The stops Samariterstraße or Frankfurter Allee should be only a few hundred meters away from your hotel) and change to the TXL bus there. They'll drive you along the entire "tourist route" between Alexanderplatz and the Brandenburger Tor, and you'll very likely get a seat as the TXL bus starts at Alexanderplatz. The total travel time will increase to about 45 minutes.