im setting up a new iMac and a new email address for a receptionist, how do i do it?

I think you will find what you need in Add or remove email accounts in Mail on Mac - Apple Support

My equipment here, my MacBook Air keeps telling me a need a second contact email address. I only have mine and I don’t know how to set up an email let alone input it into the computer. Apple really needs to break these peoples questions apart to see what else they can help their customers with. If they’re had been an article about making an email address with this article or somewhere on the support pages at least. If a company sells something that requires their customers to do then it’s their obligation to ensure ANY & ALL instructions are understood! It’s just good business. Some of us older ones who haven’t experienced very much of this technology have a slower time catching up. When I had a computer last it was a computer, not something that looks like a small serving tray when it’s closed, so there have been many strides forward. Thank you again.