I have an iMac - do I need antivirus?

No you do not need third party Anti-Virus, Protecting, Cleaning software or a VPN installed on your Mac(s) or iPhone(s).

macOS already has all of the protection and security that you need built into the system.

see > Block pop-up ads and windows in Safari - Apple Support

and > Protect your Mac from malware - Apple Support

and > macOS - Security - Apple

No, please don't install that on your iMac. It will basically become a spambot that constantly tries to upsell you other services. It will also slow down your iMac with unnecessary processes constantly running in the background. One of the best things about owning a Mac is the fact you generally don't have to worry about viruses. It is very rare for a Mac to get any virus unless you are constantly hitting dodgy sites and installing random unsigned software. I've never had a virus on any of my Apple devices over 15 years.