You can either take the X9 bus to Ernst Reuter Platz and then the underground U2 to Nollendorfplatz and walk the better part of a kilometre from there or you could (should) take a taxi which is much more convenient and will cost less than €20. With bags, it's probably the best option. Or you downtown the myTaxi app and register your credit card. Order a taxi as you collect your bags and it'll be there for you when you exit the terminal (one of the great advantages of this ancient airport is that the direct distance between the plane door and the taxi stand is about 45 metres.) PM me if you want more info, I'm in Berlin pretty much every other week.