how do I scan my computer for viruses?

No Anti-Virus software or so-called “cleaning” apps are needed or recommended for Mac OS. They can conflict with Mac's own built-in security. At best they will slow your Mac by using unnecessary resources and at worst will bork your entire system. Your Mac has all the built-in protection you need.

macOS - Security - Apple

Your computer does not have a virus. There are no known Mac viruses out in the wild for your computer to get.

Effective defenses against malware and ot… - Apple Community

If you suspect you have adware or malware you can run MalwareBytes to help you find them or you can run etrecheck to identify and help remove unwanted items.

Phony "tech support" / "ransomware" popups and web pages

If you suspect you have installed adware/malware. Try running this trusted utility https://www.malwarebytes.com/mac/