A1224 lock screen pw forgot how to reset

I was gifted a Mac A1224 20" desktop or iMac and has a lock screen password on start up. How do you reset it I've tried with the windows keyboard holding down the Ctrl+r and windows+r alt-r which is suppose to be the default command key as it boots up but doesn't take me toa screen where I can erase all disc data then reinstall software? How to force start in recovery mode with keyboard and not just holding power button.