Turn the headlights on; and have someone watch their brightness while you try to start the motor. If the headlights dim way down, or go out when you try to start; disconnect both battery cable clamps and scrape the inside of the clamps and the outside of the battery posts with a tapered reamer type battery cable service tool. Reinstall the clamps and tighten the bolts until the clamps cannot be moved by hand.
If the headlights dimmed or went out in the above test; and cleaning and tightening the cable clamps does not help; have the battery fully charged and then tested with a load tester (not just with a voltmeter). If the battery fails the load test; it must be replaced. If the battery works after being recharged; have the alternator tested.
If the headlights did not dim in the test; the neutral safety switch or the starter solenoid may be defective. If you can jump power to the starter solenoid and it then cranks; the neutral safety switch is bad. If you jump power to the starter solenoid and it still doesn't crank; the starter solenoid should be replaced.