I bought a used Mac and it was not wiped. how do I wipe it?

Here is: What to do before you sell, give away, or trade in your Mac - Apple Support

That’s the recommended procedure, though you will not be able to sign out of an Apple ID controlled by another.

The person with the Apple ID and password can do those sign-out operations remotely, too.

Here’s a direct pointer to booting Recovery: How to reinstall macOS - Apple Support

I’d use Option-Command-R to get the most recent macOS version supported by the Mac.