In Malmö there is one bus company called Skånetrafiken, their buses travel within Malmö, other cities in Skåne and a couple of other cities in Sweden but also over the bridge to Copenhagen!
All the green buses are buses that travel within Malmö and the yellow buses travel outside Malmö.
Skånetrafiken has a couple of customer centers one of them is at Centralen and the other one behind Triangeln. There you can buy buscards, ask questions and get information.
But Skånetrafiken also has several service agents in Malmö where you only can buy buscards or extend their validity.
They have service agents in almost every "Pressbyrån" in Malmö, Pressbyrån is a store and is located at Södervärn and Gustav Adolfs torg.
For an adult it costs 18 SEK for one way, but then the ticket is valid for one hour.
For a child it cost 11 SEK and the ticket is valid for one hour.
From 1st of June 2011 the buses will not receive cash on the buses so you have to buy a buscard.
My suggestion is to buy a "discount card" which costs at least 200 SEK (you can buy for more money if you want to). This card you can use whenever you want and you even get a discount of 10 % when you use it on the bus.
But for students that have a student identification I would suggest the "leisure card" which you buy for a month ahead and its costs 120 SEK. This card you can use between 17-06 on weekdays but during the whole weekend you can use it whenever you want to.
The difference is that with this card you don't buy a ticket each time you enter the bus instead it works how many times as you want, but have specific hours.
Skånetrafiken's own webpage: Skånetrafiken