You can buy your ticket on board by telling the driver where you are going. One-day and weekly travel cards are available in some towns and cities, and these can be bought from the driver, kiosks, and bus stations. In some cities, like Oslo and Trondheim, it is cheaper to buy your ticket before getting on the bus.
The travel planner Entur gives you travel suggestions from door to door by bus, train, metro, boat, and aeroplane, whether you want to check when the next bus is leaving from your destination, or you are planning a holiday from Vestfold to Vesterålen. You can also buy tickets for most trips with Entur (easiest if you use their app).