Always disconnect the negative battery cable before servicing the starter. Battery voltage is always present at the large B terminal on the solenoid. When removing the starter motor, be prepared to support its weight after the last bolt is removed, because the starter motor is a very heavy component.
NEVER operate the starter for more than 30 seconds at a time. Too much cranking will cause the starter motor to overheat, causing permanent damage. Allow the starter to cool for at least two minutes between starting attempts.