1) The best overall thing to start out on, is purchasing a Multi-Monitor Computer from multi-monitors.com. Only then will you have the full capability of harnessing full range, unadulterated, no holds barred Multiple Monitors. They are the masters of the Dual Monitor Setup and of Multiple Monitor computers. With they're own brand rightfully coined as Super PC, they carry a Dual Monitor Adapter that really kicks butt for a little plug and play gadget that anybody that has an extra VGA outlet can use as a Dual Monitor Cable. Best of all they offer the best quality with the best price, and are some of the original pioneers for this new breed of multi-tasking super powers.
2) If you get a USB to VGA Adapter, or the USB to DVI Adapter (links above) that will allow you to add an extra monitor to your computer via any USB 2.0 port. You can also add multiple extra monitors by using multiple adapters however not recommended over a Dual Monitor Video Cardor more. This is a great option for viewing documents, surfing the web, using Microsoft office and most all those other business applications that can tend to slow down productivity by forcing the user to toggle between windows on a single monitor display. however, this may not be the best application for intense graphical situations such as HDTV, Blue-Ray, Gaming and 3D/CAD Workstation applications and so forth. For those types of applications, its highly recommended that you purchase a high-powered Multi-Screen computer from multi-monitors.com.
3) You can replace or add an extra video card to your existing computer, depending on how many monitors you want to support. Although this may sound easy to do on paper, in reality it can be complex solution for a newbie/novice beginner, especially when trying to find a compatible graphics card to support things such as Dual Monitor Wallpaper and other Dual Monitor Backgrounds without having to install Dual Monitor Software.
Once you have your Multiple Monitor System set up and ready to abuse, you will need to enable all of your monitors in your "Display Properties" in the Control Panel Section. Here is a splendid link to a virtual Multiple Monitor Setup in the Windows Control Panel.
A few other options may include getting a Multi Monitor Video Card and go with Multiple Monitor Displays. It make much difference in the setup in Windows Control Panel if you have a Multi-Monitor Display over a Dual Monitor Display like this. For example if you were a stock guru that uses a Trading Computer to support your Multi Monitor Setup then obviously plugging in an adapter or two may not be the most efficient way to ensure those stock quotes are humming along as fast as you would like. The refresh rate may lower with the adapters. For laptop users check out these Multiple Monitor Laptop Systems.