If you are traveling by bus in the RNN area, you can buy tickets from the ticket machines or directly from the driver at the front of the bus. These tickets are valid for immediate travel and are therefore already validated. Monthly tickets or a Deutschland-Ticket are not available on the buses.
The buses are equipped with card readers that recognize all standard payment systems. This allows you to pay for your ticket conveniently without cash. If you're short of time: simply get on and buy a ticket.