A dimmer switch is great for taking control over the way a room is lit.
It gives you the ability to make subtle changes in light levels—to set the mood in a dining room, lower the levels in a child’s room, or even give you flexibility with outdoor lighting. In addition to these aesthetic benefits, a dimmer also has a couple of important, practical plusses. First, a dimmer saves energy by allowing you to tailor electricity usage very precisely, eliminating waste. It also extends the life of most light bulbs because it minimizes or eliminates the "filament shock" when you flip on a conventional light switch.
Replacing a conventional light switch with a dimmer switch is a relatively simple task, even if you're only a moderately experienced do-it-yourselfer. Note: If you intend to use fluorescent bulbs in the lighting circuit, you will need a special dimmer that is designed for this purpose.
Just remember the cardinal rule: Always shut off the power to an electrical circuit before working on it. Turn off the circuit breaker at the electric service panel, and then refer to the following for step-by-step procedures.