My iMac keyboard died, I aquired a working used one, How do I get them to sync?

I am trying to connect a different apple keyboard to my iMac. How do I get the iMac to recognize this new device? The keyboard turns on, but I can't type with it when sitting in front of the iMac. Do I need the mouse to be working too? (I have been using a Logitech wireless mouse and keyboard until I was able to get this apple)