Do I have to turn-off WiFi on my iMAC first?

First before what?

If you mean that you are doing the procedure in the page that you posted from ( What to do before you sell or give away your iPhone, iPad, or iPod touch - Apple Support ), there is nothing that you need to do with your wi-fi or your iMac.

If that is not the question, please reply with more information.

Bernard ..

Before doing what?

if you do the steps from the page you came from the password to your wifi network will no longer be on the mac so it will not be able to connect to it no matter if you disable wifi or not