Check first the battery chassis connection and that to engine casing itself - usually flat braided high current cable.
If all ok there are two other possibilities. If you only hear a faint click, from the starter relay it could be this faulty or complete failure of the piggy back starter solenoid. You can check with a voltmeter whilst someone turns the key. If you hear a loud clunk this indicates the solenoid is trying to engage the starter pinion but not supplying current to motor underneath, or the starter motor itself has failed.
