If it's not turning over, it's probably a loose connection, a sticking solenoid, a bad neutral safety switch, or the ignition switch. Cleaning the battery connections would be a good start. The only way to really locate the problem is to trace the circuit with a meter.
I would start at the starter solenoid and see if the small wire going into it is energized when the ignition is turned to the start position. If you're not getting voltage to the solenoid, it isolates the problem to the ignition, switch, or neutral safety switch. If you are getting juice to the solenoid, the issue is with the solenoid, battery, or starter.