Following information will help you to use Computer Monitor as TV:
First of all, you need cable box with DVI or HDMI output. If you have a cable box with an HDMI output you will need an HDMI to DVI Cable (If your monitor has HDMI input you don't need it, but most do not have that facility). Next you need speakers that accept RCA audio cables (or digital, whatever kind of sound cable your cable box supports).
Monitor should be HDCP Compliant (High Definition Content Protection, think TV DRM). Next, it must have a DVI or HDMI input. Also, make sure that it can display at Least 720p.(Use this image as reference).
Doing Set Up
The basic set up is as follows:
One of the cable from cable box will be connected to sound box of your system and other cable (DVI or HDMI) to the monitor.
After Set Up
After setup, if you don't see any picture on your display, your cable box is probably trying to display a resolution which your monitor can not display. Find out how to enter HD setup mode, or change the maximum resolution (Get help of information manual). After you find the instructions, follow them to change the maximum resolution to the resolution of your monitor (most likely 720p). Also, if the cable box says your monitor is not HDCP compliant, either you bought the wrong kind of monitor, or it is like my setup which flashes that message everytime I turn on the monitor, but only for half a second or so.