how do i stop my mixer tap from leaking ?

when i turn on the tap on my mixer tap water leaks out from the bottom of the shaft.
do i have to replace the washer on the tap or is there a washer at the bottom of the shaft
. when you swivel the shaft one way it tends do leak more than when you turn it the other way.
so is it the tap or the shaft washer that needs replacing.
if its the one at the bottom of the shaft how do you get to it.