As stated earlier, an accurate monitor profile is the basis for an accurate color-managed workflow. You can either calibrate your monitor by eye or by using specialized hardware.
When profiling your monitor, leave it switched on for at least 30 minutes before starting calibration. To perform the calibration, follow the instructions provided with your choice of tool. Note that room lightning, the color of your walls and desktop, and even your clothing can influence calibration results.
Before starting to calibrate and profile your monitor, make sure you set the display color depth to at least 24 bits.