Buying train tickets from Copenhagen to Kalmar

You can buy them in advance at www.sj.se or from a ticket machine at the station in Copenhagen.

Copenhagen to Kalmar is on the Oresundstag commuter train. So not something you should worry about now. You can, as indicated, just buy a ticket at the station and travel. Prices are fixed.
You can buy your ticket in advance on https://www.oresundstag.se/, which is the website of the company that runs the trains, or on www.sj.se, but those sites can be tricky is they aren't really geared to tourists.

Thank you

Mike
DC

Skånetrafiken's ticket machines (close to the exit to track 1-2 in Copenhagen) or app is cheaper. There is also group discount starting at two people.