Berlin Tegel airport

Take the TXL bus to Berlin Hbf and transfer to S5, S7, or S9 to Hackescher Markt. If you have an iPhone, I recommend the Berlin City app. It's free, doesn't use data once you download it (aka no need to be connected to WiFi), and it was a GREAT tool for navigating public transit when I was in Berlin.

Ellis, When you take the S-Bahn at Hauptbahnhof S-5, S-7, or S-9, as correctly pointed out above, take the one going east, not westward. Look for Alexanderplatz, Ostbahnhof, Ostkreuz as guides with the terminus for S-5 Strausberg Nord and Ahrensfelde for S-7.