Kate,

This is usually really easy. There are books in your local library on
home repair with lots of picture that may help you.
Turn off the power to that light at the breaker box or fuse box. Remove
the screws that hold on the cover plate at the switch. Remove the cover
plate. Remove the screws that hold the switch to the junction box. Pull the
switch out of the junction box. Look at the wires attached to the switch
(often black, white, green). Draw a diagram of these wires and the switch
and then disconnect them. Take the switch to the hardware store. Find one
like it. Reverse the above to install the new switch.
There will be instructions on the package of the new switch which may
help you. If you can't find the same type then you can substitute another
type. The wiring will be essentially the same but you must buy a cover plate
to fit the new type of switch. Note that the color of the switch and plate
should match and should match the rest of the room's cover plates (white,
ivory, brass, et c.). After you attach the wires to the switch neatly fold
the wires into the junction box so that the switch will fit.
You'll need a flat head screw driver and maybe a set of needle nosed
pliers.

Good luck,
Dave M.