Start by using a volt meter connected between the small terminal on the starter solenoid and turn the key to start and see if you have 12 volts. If 12 volt then I would suspect starter if this is the 3 terminal solenoid. If four terminal and you have 12 volts at the solenoid then start checking the pto switch, the safety switches on both levers, and the parking brake switch. On some more also need to activate the seat switch to start. If you have the three terminal solenoid and don't have 12 volts then go through all the safety systems.