How do I remove a virus from my iMac

I have gotten a virus in my iMac.

No you haven't. What you got is a scam alleging you have a virus in your Mac in an attempt to defraud you. You have a Mac, therefore you have money (lots and lots of money) meaning crooks of all stripes want to get their hands on it.

There are no viruses that affect macOS. Just forget that idea.

If Stop unwanted Notifications - Apple Community does not describe the problem then go ahead and run EtreCheck as rkaufmann87 suggested.

Do nothing else until directed. Install nothing. Do nothing. Wait for instructions.

Andrew,

First the good news! There are no virus's for Mac OS so you do not have a virus. In order to try to figure out what the problem might be please do the following:

It will help us get a good idea of what is causing your problem if you provide a report of your system, that way we do not have to play 20 questions with you. The report we are requesting does NOT provide any personal information and is extremely safe to use. Please navigate to www.Etrecheck.com and download the free version of EtreCheck. Once you have you downloaded the app and installed it, please run the report and save it. This report will help us get a good idea what has been installed on your system and help us be able to diagnose what may be wrong.

When you have your report, you can attach it when you reply to this message and we can then review it and help you determine what is needed to get your system running well again. 

For instructions on how to download your EtreCheck report and attach it to your reply to this message please click How to use the Add Text Feature When Post… - Apple Community It is helpful to us if you download the report to your Desktop.