* How many wires do you have comming out of the ceiling? There should be one white and one black (or red) but no more than two wires, and the copper ground wire.
* From the light fixture there should be two wires, one black and one white.
* check voltage from white to black. It should be hot when switch is in on position and no voltage when the switch is in the off position.
* Possibility: The switch is shorted. It is always on no matter the position of the switch.
* Possibility: Wires on the switch are touching.
* Good luck... Adios... Jasper...