Open Control Panel, choose Hardware and Sound, then under the Display heading, choose "Connect to an external display."
Connect your second monitor to the computer. The Display window will flicker (or go blank for a few seconds), then you'll see a dual-monitor setup screen like the one shown here. If you don't, click the "Detect" button. If that does not work, check the monitor's connection to your computer.
Once the second monitor is detected you need to choose how the two monitors will operate. There are three options:
"Duplicate these displays" shows the same display on both screens. This is handy when monitoring a presentation on a laptop while it is displayed on second, larger screen. "Extend these displays" makes one big screen out of the dual monitors. You can actually drag objects across the boundaries of the two monitors. This is the setting that most people use, and the one I recommend. "Show desktop only on 1" or "Show desktop only on 2" disables one of the monitors.
Adjust the screen resolution of the monitors as desired, press Apply, and you're done.