The topic of monitor calibration can be complicated, and if that's your interest, try an internet search. A plethora of sites offer exhaustive and free information to satisfy every possible query on the subject. You can also refer to the manual that came with your monitor. For our purposes, let's keep it simple.
Accurately calibrating your monitor is very important for color-critical work. Such a monitor should be calibrated often, which in turn promotes predictibility in how an image is displayed.
The main purpose of calibrating is to set white and black points, contrast, brightness, and gamma (mid-tone density). In a matter of days or weeks, your display will drift out of calibration. This is why we recommend calibrating your monitor at least once a week.
To calibrate your monitor, you'll need software. Adobe Gamma (supplied with the Windows version of Photoshop) and Monitor Calibrator (MAC OS only) are simple to use. Both programs have "wizards" that can guide you, step-by-step, through the process. A variety of more sophisticated software can be purchased from third party developers, as well as high-end software that is included with the purchase of a monitor specifically designed for color-critical applications.