It could be the solenoid or the start circuit. To check, pull off the wire at the starter solenoid and use a test light or voltmeter to check for power on that wire when the key is held in crank position. It should have power- if it does, likely the solenoid is bad. If no power on the solenoid start wire, then problem is in the start circuit between the ignition switch and the solenoid-check the neutral safety switch, check for power out of ignition switch on the start wire, with key held in crank position, check starter relay for proper operation. All these items are in the starting circuit.