Train service from Gothenburg to Stockholm

So im flying to Sweden this Friday for my first trip to the county. I’ll be arriving in Gothenburg, but I’m planning to go straight to Stockholm from the airport to catch up with some friends.i tried looking on the SJ website but couldn’t really figure it out. Can I just buy a train ticket at the station in Gothenburg, or would it be cheaper/easier to book it before hand? And around how much does it usually cost?