From the central bus station (ZOB) you can take the underground U2 which will take you directly into the heart of Berlin (both western and eastern city). If you want to go by bus, there are the lines M49 and X34 which will take you to Zoologischer Garten Station. There you can change to S-Bahn, underground and regional trains to further progress into the city.
The best choice to get to Berlin-Spandau by bus is line M45 which leaves at Zoologischer Garten station and takes you directly to Rathaus Spandau. Faster options are underground U7, the S-Bahn S3 and S75 and the regional trains RB14 and RE2.
Remember: The former line 92 is now called M37. There are two northern and southern branches of M37 and none will follow the whole former 92 route. So if you want to explore the "real" line 92, you'll have to change at Rathaus Spandau (for the correct southern branch bus will take the wrong northern branch and vice-versa). From Rathaus Spandau to the north: M37 Spandau Freudstr., from Rathaus Spandau to the south: M37 Reimerweg via Sandstr.
Day dicket is at 6,30 EUR, 7 day ticket costs 27,20 EUR
For more information please have a look at www.BVG.de
which is also available in English.