Connecting wireless keyboard to Mac

I bought a Mac from a friend and it’s completely fine, the problem is to activate the Mac I need to connect it to WiFi, and to do so I need a keyboard. The friend who sold it to me gave me their keyboard too but I can’t connect it because there’s no Bluetooth button anywhere. I’m on the first screen when you get a Mac, there’s a message that says I need to connect to WiFi and then there’s the Apple logo. When you click it three options appear: Startup Disk, Restart and Shut down, no Bluetooth button. Any help appreciated.