Attach a black lead of a voltmeter to the negative post of the battery and the red lead to the positive and try to start the engine, what is the voltage.
Now move the red lead to the battery side of the starter solenoid, and try to start what is the voltage.
Move red lead to starter side of solenoid and try to start, what is the voltage.
Move to starter and try to start, what is the voltage.
Now put the red lead on the housing of the starter and try to start, what is the voltage.