Before you work on any images you need to characterise and calibrate your monitor to a visual standard that suits the work that you are doing. Otherwise the image on your monitor may look quite different when displayed on another calibrated monitor. The following has made the assumption that you are working in Photoshop.

There are 2 basic ways of calibrating a monitor. The best (and needless to say the most expensive) is to use a device that reads the phosphors and their transmissions from the monitor. Probably the most widely used brand would be the X-Rite. The other and almost as good is eyeball calibration and the Adobe Gamma utility (no cost as it is included in Photoshop).

In Windows, start Adobe Gamma, this is located in the control panel folder or in the Program files/Adobe/Photoshop x/calibration folder on your hard drive. In Mac OS, from the Apple menu, choose Control Panels > Adobe Gamma.

Do one of the following

To use a version of this utility that will guide you through each step, select Step by Step,and click OK. I recommend this version if you are inexperienced. All you do is follow the instructions as displayed. Start from the default profile for your monitor, if available, and enter your own name for the profile. I would recommend the name of the monitor and date you did this. When you have finished with Adobe Gamma, save the profile using the same description name. If you do not have a default profile you may be able to get one from the monitor manufacturer or their website.
To use the compact version of the utility with all of the controls in one place, select control panel, and click OK. This version is great if you have experience in creating colour profiles.

For more information on this topic go to the Adobe site and search on ICC profiles.