I am providing you with a wiring diagram of the starting circuit.
If you will notice tracing from ground back to the ignition switch, the first thin g to look at would be the grounds and the Anti-Theft Relay. If so eqipped. If you have an automatic transmission, the thransmission range switch is a strt sicrcuit safety feature. It only allows the start circuit to complete if the car is in neutral or park. So it could be bad or the shifter might need adjustment to make sure it is going exactly into each gear detent as it should. Failure to go into the detent could make the circuit fail and the starter would not turn over.
Then there is the ignition switch which can fail internitantly. The battery distribution box has a ground that should be checked. Then the battery cables. The negative cable has two grounds to check. The positive cable goes to the starter and the alternator. Finally the circuit comes back out of the starter to connect with the anti-theft system by means of the dashboard electrical distribution box.