More than likely, it's either the starter relay(solenoid), or the ignition switch. Turn the key to the "on" position, then jump the starter from the positive terminal (from the batt cable side of the relay, to to the small 'ignition' post on the relay). If the car starts normally, and the starter doesn't run on like it's been doing, it the ignition switch. If the problem persists, it's the relay.
