Will not crank

The engine will not crank, no sound of turning over when ignition is turned.the battery checked at twelve plus volts. Also, checked turning headlight on and off and turning ignition switch the voltage never got under twelve volts. Pulled starter and took was told nothing was wrong with it. Put meter on solenoid wire to starter and it read 12.3 volts fluctuated some while trying to crank. is there anything else that it could be? I am borrowing this vehicle so I do not want to mess it up, but really cannot afford to take it to a mechanic. I have noticed some days it hesitated to start, but I figured It did not have the clutch engaged fully. it always fired up the second or third time. The truck also revs up on its own. was checked for vacuum leaks and none was found. please help with any advice. thanks in advance.
Rebecca