Your working environment should have enough light and a minimum of glare to let you see the keyboard, screen, and nearby papers and documents but not so bright that the contrast between the screen and the rest of the room makes text on the screen difficult to read.
If possible, the room where you work on your computer should have a combination of well-distributed ambient light and direct task lighting (usually from a table lamp) aimed at the keyboard and the surrounding table. Place the light sources in a position where you can't see glare or direct reflections on the screen. The monitor screen provides its own light, so it's not necessary to point a task light at it.
If there's a window in the room, the best location for the computer screen is perpendicular to that window, so the bright outside lighting during the day and the darkness at night doesn't produce a distracting contrast to the amount of light coming from the screen. If the monitor is opposite the window, the sun can shine directly onto the screen, which makes it extremely difficult to read text or view images. Curtains, window shades, or blinds can often help with this.
If lights in the room use fluorescent bulbs, the flicker on a CRT screen can become very distracting, especially if the refresh rate is set to 60 Hz. The display is far less unpleasant to use if you increase the refresh rate to at least 70 Hz. LCD monitors don't have this problem, so the native 60 Hz refresh rate is usually the best choice.
Use the controls on your monitor to adjust the image to the best combination of brightness and contrast in your particular environment. If the room becomes dimmer or brighter, or if the amount of light coming through the window changes, adjust the monitor settings to a comfortable level for the new conditions.
Don't forget to clean your monitor screen frequently. There's no excuse for looking at the screen through a layer of dust and grime that reduces both clarity and brightness.