Does it work on high? If so, then it is probably the resisitor pack. This cuts the voltage down to the motor on slow speeds. In high, it gets all 12 volts.
The pack is usually in the duct by the fan motor, passenger side of vehicle. There would be 4-5 wires running to a piece of plastic or metal, which is screwed to the duct.
The fan is usually easy to take out as well.
It could be the switch itself, but check it on high right now.