Dimmer switches

The switch you have is a "3-way" switch. It is used when you want to controll the light from two locations. What you want is called a "single pole" switch. However, you can make the 3 way work. You will notice that of the 3 screws on the switch two of them are the same color and one is a different color from the other two. This different colored one is called the "common" terminal.

Of the two wires that you have one is a "hot" and has power on it all the time, the other is a "switched leg" it goes directly to the light fixture.

To make the 3 way switch work like a single pole, attach the "hot" wire to the "common" terminal, and the "switched leg" to one of the two screws that are the same color. If the first one doesn't work then the other will. (or change the position of the switch)

A 3 way switch switches or "throws" from the common terminal to one of the other two screws normally called "travelers", when you change the position of the switch it throws from the common terminal to the other traveler.

Hope that helps and good luck. Make sure to turn the power off before attaching the switch to the wires. Never install a dimmer on a hot circuit, it's a great way to ruin them.