After you turn off the circuit's power and pull the existing switch from the box, snap digital photos of the wire connections before disconnecting them. Then purchase a dimmer switch (single pole for one switch or a three-way if there are two or more) that is suitable for the type of bulb you use. Make sure it's also rated for the lights you have on the circuit: Add up all the wattages and buy the lightest-duty unit that will cover the sum plus 10 percent. That's because the higher the wattage rating, the larger the switch, and the high-power ones are often too big for old electrical boxes. With the snapshots as your guide, attach the new switch's leads to the house wiring using wire nuts and electrical tape.