This app was mentioned in 6 comments, with an average of 2.50 upvotes
/r/berlin
Or might as well tell you: either at a BVG-terminal at the airport, from a vending machine, from an official kiosk, from the busdriver or by app: https://play.google.com/store/apps/details?id=de.eos.uptrade.android.fahrinfo.berlin&hl=en
The BVG app can do routing and allows you to buy tickets in app: https://play.google.com/store/apps/details?id=de.eos.uptrade.android.fahrinfo.berlin
Like others said, you can also just use Öffi or even google maps to do the navigation and buy the tickets in the station.
If you plan to use it a lot, you probably are best off buying day tickets or even a week pass: https://shop.bvg.de/index.php/tickets
These tickets are working in all forms of transport: Bus, underground, tram and regional trains.
Lad dir doch die App runter, kannst mit Kreditkarte oder Lastschrift bezahlen und brauchst überhaupt kein nerviges Papier mehr.
https://play.google.com/store/apps/details?id=de.eos.uptrade.android.fahrinfo.berlin
Gibt's bestimmt auch für Apple.
If you do not want to use any cash and are afraid your card may not work, get the BVG app and buy the ticket in app: https://play.google.com/store/apps/details?id=de.eos.uptrade.android.fahrinfo.berlin
You could try the app of the public transport organisation.
Fahrinfo from the BVG itself? https://play.google.com/store/apps/details?id=de.eos.uptrade.android.fahrinfo.berlin