Check out the back of your tower where your current monitor cable is... It is quite likely that it will have a second or even third port right next to it. If so, you just need to connect a second monitor to one of the open ports with the proper cable.

If you take a nice clear pic of the open ports (smart phone is great for this) you'll have all the info you need to go to Best Buy, etc. for a monitor and cable... Like others said, it's a matter of configuring your monitor through computer settings...

good luck -jim