Look on the back of your computer. There will either be two blue ports, a blue port and a white port, or two white ports. Your current monitor is taking up one of these (Usually the blue one if its a computer monitor or white one if its a small TV screen).
Get the other monitor and look at the end of its cable. If it matches one of the ports, plug it in. Then Right Click your desktop, go to Properties, and go to the Settings tab.
There will be two boxes representing your monitors, 1 and 2. Click on 2, and then check the little box at the bottom labelled 'Extend my Windows desktop onto this monitor'.