Go to the shop and buy a new complete switch - while you're there you can get a new cord and a fancy knob if you want.
Isolate the electricity to make it safe - take down the old switch and fit the new one - BOTH wires are Live - one is live all the time and the other one is live when the switch is on - they fit in any way round.
If you're not in any way confident about doing this then get a sparky in to do it for you. In the UK some electrical work in bathrooms is notifiable to the local council - replacing an existing light fitting does not fall into this category.
Good luck... take care...
Peccavi
October 2009