attach meter to both terminals on the battery, and attempt to start. What is the voltage.
Move the red lead to the small terminal on the solenoid, and the black still on the battery ground. attempt to start, what is the voltage.
Now move the red lead to big wire on the starter. What is the voltage.
Last put the red lead on the metal part of the starter, what is the voltage.
post back your results, and we will know were the problem is.