Proper monitor calibration is important when editing or viewing images. Monitors are not created equal, some being vastly better than others.
When you first plug in your new monitor you can bet money
that it won't be accurate. It will be nice and bright,
nicely saturated, but not accurate.
Colour and brightness sell, but colour accuracy is not at
the forefront of the most people's minds when they pick up
a new monitor.
There are already many resources on the internet telling
you how to calibrate your monitor. They're all good, and I
don't wish to duplicate the information. I do recommend
that you let your monitor warm up for at least 20 minutes
before you attempt to calibrate it.
For good information on calibrating your monitor try these
Alternatively you can purchase a calibration tool
specifically for the job. One such tool is the Spyder from
Colorvision. This is the tool I use for colour calibration
of my monitors. Check out their website here http://spyder.datacolor.com/