You calibrate you monitor by using a spectrophotometer like the Display 2 from GretagMacbeth, ColorCal from Spyder, etc, etc. A calibration unit will cost about 200-300 USD. But most important is to have a monitor that can be calibrated by hardware, if you want to have reliable color fidelity. If you are willing to make a compromise in color fidelity and quality, you can use a good midrange monitor that doesn't have hardware calibration and get acceptable results. That's what I do. I use a Philips W200W6, and calibrate it with a GretagMacbeth Eye-One Display2. I have good results. Of course, not up there with the Eizo screens or top end NEC, but good enough for my purposes.
There are "optical" methods for calibrating a monitor, but the human eye is not a reliable tool to judge color. You might get acceptable results, but you will not have a "calibrated" monitor.