It won't turn over

Try this procedure:
Check the battery like this:
Turn on the headlights, then try to start the car.
If the headlights do not dim or go out, then the battery is likely ok.
Check that the battery cable plus post, goes to the starter, Minus post to a good ground, like the engine.
If wiring is ok, then look at the starter solenoid for a good connection, and proper voltage at the starter.
Problem is most likely the Solenoid's bendix gear or starting relay, Have someone bang on the starter while you try to start the car. This usually works by dislodging the bendix gear..
Hope this helps.

If you mean the engine won't crank, check for voltage at the battery, neutral safety switch, key switch, and starter.
If the engine cranks but won't start, you will need to figure out if it is a fuel or spark problem.