The simplest and least expensive way to calibrate your monitor is to use a calibration device. This tool looks like a computer mouse and hangs from the top of your monitor while software tests and adjusts your color values. Simply plug it into your computer, install the included software, and follow the prompts. There are a number of decent calibration tools available, but these two are my tried-and-true favorites.
X-Rite EODIS3 i1Display Pro ($200)
This is my #1 choice based on quality, affordability, and ease of use. It includes options for manual adjustment and customization as well as profile options for multiple printing methods.
X-Rite CMUNDIS ColorMunki Display ($125)
This is a slightly less expensive and simpler model. It uses the same software as the i1Display Pro but offers fewer customization options. This is a great tool for anyone new to monitor calibration.