An overview of public transport (U-Bahn and S-Bahn) in Berlin can be found here.
Coming from the north – Hamburg/Rostock (A24 autobahn):
At the motorway junction Oranienburg take the A111 to Berlin Zentrum, and at the motorway junction Charlottenburg turn onto the A 100. Leave at the Kaiserdamm exit, and turn left at the end of the slip road to turn onto the Kaiserdamm Boulevard (later Bismarckstrasse). Continue over the traffic island into Strasse des 17 Juni, and the Main Building of the TU Berlin is on your right after 100 yards.
From Hannover (A2) and Leipzig, Nuremberg:
Take the motorway A10 (Berlin Ring) to the Drewitz motorway junction, then the AVUS (A 115) to Berlin-Zentrum. At the Funkturm follow the signs to Wedding, but then take the first exit to Kaiserdamm. Turn left at the end of the slip road to turn onto the Kaiserdamm boulevard (later Bismarckstrasse). Continue over the traffic island into Strasse des 17 Juni, and the Main Building of the TU Berlin is on your right after 100 yards.
At Central station take a regional train or S-Bahn (any line in direction of Charlottenburg, Westkreuz, Potsdam or Spandau) to Zoologischer Garten. The main campus is only a five minute walk from Zoologischer Garten railway station. You can also take a bus (M45, 245 oder X9) or the U-Bahn (Line 2) to Ernst Reuter Platz (or you can stop a taxi and take a fixed rate short ride from Zoologischer Garten).
Berlin has (currently) two airports in operation.
Arriving at Berlin Tegel (TXL)
The airport is very close to the Main Campus. Take the airport shuttle bus (X9) to Ernst-Reuter Platz (15 minutes) or take a taxi (10 minutes).
Arriving at Berlin Schönefeld (SXF)
This is on the other side of the city, so a taxi is expensive unless you can share the costs. At S-Bahnhof Flughafen Schönefeld take a regional train (RE7, destination Dessau) or S-Bahn (Line 9, destination Pankow, change at Ostkreuz to any train in direction of Zoologischer Garten). Leave the train at Zoologischer Garten. From there you can walk down Hardenbergstrasse (five minutes), take a bus (M45, 245, or X9), or take the U-Bahn (Line 2, destination Ruhleben) one station to Ernst Reuter Platz (Or you can stop a taxi and take a fixed rate short ride).
Another option is to take a 171 or X7 bus to the underground station Rudow and from there take the Line 7 (destination Rathaus Spandau) to Bismarckstrasse, then change onto the Line 2 (destination Pankow) and travel two stops to Ernst Reuter Platz.
Both options take about an hour.