Not sure the exact procedure for win8 but here's what you do for Win7.
Shut down the PC and locate the unused HDMI/DVI/VGA ports on your motherboard or graphics card(use the card if you have one,obviously) and plug the monitor into its corresponding output.
Power back on the PC and the 2nd monitor should be working in Clone Mode(Mirror images on both displays)
Once you get to your desktop navigate to your windows display settings(Control Panel\All Control Panel Items\Display\Screen Resolution), select your primary and secondary monitors then set "Multiple Displays" to "Extend these displays"
Like so
Then click apply and you should be ready to game!
As for games, you'll want to run them in "Borderless Window",or some variation of it,so you can freely move the mouse between the gaming monitor and the 2nd monitor.