I bought a iMac second hand and it has a password on it

On any used Mac you should erase the drive and install a fresh copy of macOS and upgrade it to the latest version, if needed.

How to reinstall macOS from macOS Recovery - Apple Support

If the computer has a Firmware Password set, you can't fix it.

How to set a firmware password on your Mac - Apple Support

If it is a firmware password that you need to enter, refer to the If you forgot your firmware password section in Apple's article, How to set a firmware password.