Final answer:
Report a bus fire to the bus driver first, as they are trained to handle such emergencies and will contact the necessary services, including police and fire department.
Step-by-step explanation:
In the event of discovering a fire on a bus, the first course of action should be to inform the bus driver immediately. The bus driver has the training and protocols to follow for such emergencies, which might include evacuating the passengers in a safe and orderly manner and contacting the emergency services if needed.
While the passengers certainly need to be aware of the situation, it's the driver's responsibility to manage the scenario and ensure everyone's safety. The police and fire department are indeed critical to handling a fire, but typically, the bus driver will be the person to alert them. Involving the police or fire department directly can be the next step if the driver is unable to act for any reason.