I bought a used Mac and have no password

NO! Never give out your passwords to anyone.

If the buyers give you the Mac you can perform the steps as listed in the Apple article @BobTheFisherman linked especially if you have your AppleID tied to that Mac in any way.

Otherwise, have the new owners perform a clean install of macOS by first erasing the whole physical drive (Intel Mac, with M1 Macs you only erase the Container) before reinstalling macOS by following the last step in the article @BobTheFisherman linked. The buyers needs to do this by booting into Recovery Mode (Command + R), Internet Recovery Mode (Command + Option + R), or from a bootable macOS USB installer.

By not erasing the old Mac you risk having your personal data accessed by other people so hopefully you had Filevault enabled on that Mac.

You have to do this: What to do before you sell, give away, or trade in your Mac - Apple Support