you should already have one in the cam.
You would've gotten a message on screen if you didn't have one in the camera.
The one I had in mine was a little small, so I got a 32gb card. 32 is about as big as the system can handle without slowing down.
Mine was the cgo2+, but only had a 8gb card. Yours may be larger because of the 4k potential.
Don't forget to format the new card too. FAT32 is preferred format.