Starter relay , ignition switch , park neutral psition switch if automatic trans , clutch start switch if manual trans. , battery cables ,connections . If you don't know how to do automotive electrical circuit testing , your best bet , take it to a qualified repair .
How to voltage drop test starter motor circuit
Starter Voltage Drop
Free wiring diagrams here
http://www.bbbind.com/free_tsb.html
Enter the vehicle info. year ,make , model and engine size . Under system click on engine ,then under subsystem click on starting . Click the search button ,then the blue link . Look an see what all is involved in the starting system . Use a volt meter an test to see where B+ voltage is an where it isn't while someone turns the key to start . You can rule out the load (starter ) side by jumping pins 30 & 87 where the relay plugs into . Watch this video on relays .
VEHICLE RELAYS Operation Diagnosis