Final answer:
Use the rear brake to hold the motorcycle when starting the engine on a hill and shifting into first gear to prevent it from rolling.
Step-by-step explanation:
To hold the motorcycle while you start your motorcycle engine on a hill and shift into first gear, you should use the rear brake. This will prevent the motorcycle from rolling backwards or forwards, allowing you to safely start the engine and engage first gear without the risk of losing control. Using the rear brake is a fundamental skill for managing motorcycles on inclines and is an essential part of safe riding techniques.