Security issue

I was in Safari and was doing a search when I got a message that my computer was compromised. It said to not shut it down and to call Apple Support right away. It gave a 800 number. I shut down my computer and unplugged it.

I assumed it was a scam.

I don’t know what to do next?

How do I know if I have malware on my computer?

How do I get rid of it?