Make a left turn from BA gates. Get your transport tickets before exiting the airport, there's a BVG desk right there with maps and all. If for some reason the desk is closed, there are ticket machines outside. Stamp the ticket on the bus (single or multi-day tickets only need to be stamped once)

Take bus "TXL" to Zoo (Zoologischer Garten) and change to the U-Bahn line U2 in the direction of Pankow. Get off at Potsdamer Platz.

Shouldn't take too long.

I did this trip for Presidents Day on BA, only I stayed on Kurfürstendam in West Berlin, so my bus ride took only 20 mins.

Enjoy Berlin .

EDIT: I stand corrected, see response below