There are three terminals on the back of the solenoid assembly on the side of the starter motor. The one closest to the starter motor is the "M" terminal. It connects to the starter motor. The middle one is the "S" terminal. It's driven by the starter relay. The last one is the "B" terminal. It's connected to the battery and the alternator.
Measure between ground (some good bare metal on the car body) and the B terminal. It should be 12.6 volts. If it is, have someone turn the key to start and see if it remains at or near 12.6 volts. If it is zero or very low in either case, then the cable to the battery is bad or there is a loose connection somewhere.
Measure between ground and the "S" terminal while someone holds the key in the start position. It should be over 12.0 volts. If it is very low then you have bad wiring, loose connection, bad relay, blown fuse, or PATS/PCM/Key issues.
If the "S" and "B" terminals had 12V, then measure the "M" terminal while someone holds the key in the start position. It should be 12 or so volts. If not, then the solenoid on the starter is bad and the whole starter assembly will have to be replaced.
If the "S", "B", and "M" terminals all had 12 volts, then measure between the same ground you used before and the bare metal of the starter motor while someone holds the key in the start position. If you get more than 0.5 volts, then the ground strap between the engine and the body is missing, loose, or bad. Otherwise, the starter is bad.
