You can buy a single ticket in the ticket machine at most of the stations. You can only pay with card - not with cash.
Single tickets are the most expensive option per ride. It can however be a good option, if you only travel by bus a few times, while you are here.
Rejsekort is an electronic ticketing system for travelling by bus, Letbanen and train.
You have to check your rejsekort in and out on the card reader located on the station. It is important to check in before boarding Letbanen.