From Tegel Airport

Busses from Tegel International Airport operate to S+U Alexanderplatz, Hauptbahnhof (bus TXL [3]), and S+U Zoologischer Garten (buses X9 and 109) for the standard ticket fare. It takes around 45 minutes to Alexanderplatz – it is much quicker to get off at Hauptbahnhof or Zoologischer Garten and change to the S-Bahn (the bus ticket remains valid for this).

From Schönefeld Airport

Take preferentially the RB7 and RB14 regional trains. These Express trains run to and from central Berlin (Mitte) every half hour from 5 am – 11.30 pm and take approx. 25 min to/ from Alexanderplatz; 30 min to /from Berlin Hauptbahnhof and 35 min to/ from Zoologischer Garten. There are two of these every hour and they leave the airport station at 25 minutes and 55 minutes past the hour. The trains are big and comfortable and run a far quicker service than the S-Bahn.

The S-Bahn S9 runs every 20 minutes from Schönefeld airport. If all goes smoothly, it will take you approx. 30 min to/ from Ostkreuz and 45 minutes to/ from Pankow, while the S45 connects to the circle-line (Ringbahn) and also runs every 20 minutes.

Taken from wikitravel.org