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.