The first thing to try is to find an empty space where you can safely drive
in reverse and then hit the brakes.  Many cars have self adjusting rear
brakes that adjust when the car is being driven in reverse.  Do this a few
times and then see if the brake is working.  If nothing changes, you need
to take up the slack in the cable.  How this is done will depend on your
car.  In some cars you can do this with an adjusting screw right at the
parking brake handle.  On others you have to get under the car and adjust
it near the rear brakes.  
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Alex