You must yield the right-of-way to a police car, fire engine, ambulance, or other emergency vehicle that uses a siren and flashing lights. Pull as close to the right of the road as possible and stop until the emergency vehicle(s) has passed. However, don't stop in an intersection. Continue through the intersection and then pull to the right as soon as you can. Sometimes emergency vehicles will use the wrong side of the street to continue on their way.
Different states have different laws regarding how far away you must stay from an emergency vehicle. To be safe, do not follow within 500 ft of a fire truck answering an alarm or an ambulance when the flashing lights are on.
In some states (e.g., California), you can be cited for driving too close to an emergency vehicle.