Testing the starter electrical circuits is how to find the problem . Viewing wiring diagrams to see what all is involved in the starting of your vehicle ? Most vehicles have a starter relay that is energized by the PCM/ECM - engine computer . B+ voltage comes from the ignition switch going through the starter relay control side to the PCM - which will ground that circuit turning on the relay sending B+ voltage to the S terminal on the starter solenoid .
Do you know what a automotive relay is an how to test ? How to test for B+ voltage at the starter ? How to use a DVOM - digital volt ohm meter ? Free wiring diagrams here http://www.bbbind.com/free-tsb
Enter your vehicle info. year , make , model and engine size. Under system click on engine ,then under subsystem click on charging system . Click the search button then the third blue link , this is the starter electrical circuit . This diagram doesn't show the park neutral safety switch input to the PCM . Speaking of the park neutral switch , put transmission in neutral an see if it starts then , could be the switch is bad .