Is this a desktop or a laptop? Most laptops I've worked with didn't support dual monitors without an external video card. The exception being business line laptops on docking stations.
For a desktop if you have the ports to connect a second monitor then you should be all set, otherwise worst case scenario you can get a $20 video card from newegg to get you a second port. Once both are plugged in, the second should just come on. In windows 8 the default (for me) has been to mirror the image on both screens. You can change this, and the side (left or right) that your new monitor is on by right clicking on your desktop and selecting the "screen resolution" option. You likely want to use the "extend these displays" option so both screens show something different. It will also allow you to drag and drop the displays around so if your second display is physically to the left of your main monitor you can change windows to match.
This link might also give you a hand if I wasn't overly clear:
Connect a second monitor or projector - Windows Help