Assuming the wiring is new enough, there will be a black and white wire going to the fan from the switch. The white wires of the switch and light are probably tied together with a wirenut. Turn off the power at the fusebox that works the fan/light combo. Then take off the switch plate and remove the switch from the switchbox. Find out which one of the black wires goes to the fan (you'll probably need to reapply power to accomplish this). This is the wire that you need to run through the new switch you will install. If there is no more room in the switchbox, then you can get a double switch which is the same size as a normal switch. Label everything if you think there is a possibility of confusion.
The black wire from the fan goes to the brass screw of the new switch. You need to get power to the other screw of the new switch. You should be able to tap the other side of the old switch (where the black wire wasn't hooked up) and jumper a wire from there to the new switch. Or you can find the source of this point and use wire nuts to jumper a wire. Be sure to use a wire of 12 or 14 gauge, whatever is already there.
edit>> in the meantime, you should be able to unplug the fan to keep it off altogether. Carefully pull down the fan cover, and there will be a plug there for you to remove.