How do I pair keyboard/mouse w/ computer without keyboard and mouse previously paired??

Hello tabard,
I understand that you would like to set up a keyboard and mouse to your iMac but didn't have anything paired to originally. That is something that I can assist you with.
The one thing to have is a USB mouse so you can navigate the menus for setting your keyboard and mouse. Take a look at the article below to walk you through once you have access to a USB mouse for the easy setup process.
Set up your Apple wireless devices
https://support.apple.com/en-us/HT201178
If you don't have one handy, you can always shut down your computer and power it back on. Being that it recognizes that there is no keyboard or mouse connected, your iMac will launch the Bluetooth Setup Assistant window. It will then prompt you to turn on your mouse and keyboard and make them discoverable.
Take care