Thumb rules

1. Lower the current in a circuit by adding resistors. Example: an LED will burn if the current is not lowered by adding large resistance.
2. Most motors can not be driven directly by an Arduino out pin as they can't supply enough current -- use a motor driver (shield) or transistors to amplify current.
3. Motors are a great source of interference that can make the rest of your electronics misbehave. This can be countered by isolating the motor power supply to a greater or lesser extent.