https://www.berlin.de/en/public-transportation/1772016-2913840-tickets-fares-and-route-maps.en.html
https://shop.bvg.de/index.php/tickets
Start at airport Tegel (18 km)
Use bus 109 towards Zoologischer Garten.
Change at Jakob-Kaiser-Platz for underground line U7 towards Rudow to Fehrbelliner Platz.
There change for underground line U3 towards Krumme Lanke to the station Freie Universität (Thielplatz).
Leave the underground station by walking to the direction of travel and by using the left exit.
The Harnack House is 100 meters away on the right side.