I have virus on my iPhone. What to do?

Can you please share how do you even know if you have virus on your iPhone? If it is from some random websites/apps that tells you that there is a virus on your iPhone. It is most likely a scam. Please ignore those. However, if your iPhone is behaving strangely, it could be sue to software issues or your account getting hacked and so on. I think you should contact Apple support at https://support.apple.com/iphone Scroll down and click on Get Support and talk to someone there.

You don’t have a virus on your phone. For all practical purposes iPhones can’t get viruses. Why do you think you do?

That Pop-Up will be a scam trying to get you to reveal important information. Check this link...

Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support

Please describe the symptoms of the virus, so we can offer relevant advice.