How to add a 2nd monitor

To elaborate on adding a second monitor to a desktop, many newer desktops come with multiple video out ports. Dell computers come with VGA and Display port and/or HDMI.
Display port is becoming very popular as you can adapt it down to almost any format; HDMI, DVI, VGA.
Also, if you have a discrete video card (add-on), it may have multiple video out ports also, including mini-display port (adaptable as well).
If you have the capability to add a second monitor already, simply plug the second monitor in (a reboot *may* be necessary) and open up your display properties (assuming you are running a Windows based computer, Mac's are a bit different) and you should be able to detect and identify your second monitor and choose how it works with your current one (mirror, extended).
As the other responder suggested, more detail in what you are wanting to accomplish, what hardware you have, and what operating system (and version) will garner much more detailed responses.

Most laptops have a VGA port so an external monitor can be added. Then you might have to press a Fn key combination to get the other monitor to display.
If you want to add a second monitor to a Desktop computer, you'll need to install another graphics card. A USB adapter card is available; do a search for different models available.

If neither of these answer your question, please explain exactly what you need a monitor to display from; you posted to the Network and Wireless Adapters section, which is confusing..