Emergency Scene Management is a series of steps designed to ensure the safety of the rescuer and the casualty. It is based on St. John's Ambulance recommendations.
The first stage in ESM is to perform a Scene Survey:

identify yourself and offer to assist the victim
if they are unconscious you can proceed with implied consent
if there is a possibility of head or spinal injury, tell the casualty not to move
determine if the scene is safe for you to give first aid or if there is a risk of further injury to yourself or the casualty
assess the number of casualties and the severity of injury using the primary and secondary survey

Primary Survey

The primary survey is used to assess if the victim has any life-threatening injuries and should be performed in the position found. An easy way to remember the steps of a primary survey is to think of the alphabet.

A-Airway is the airway clear? If the casualty can speak without difficulty than the airway is clear
B-Breathing check to see if the victim is breathing and how effective it is, if they are not breathing then begin artificial resuscitation
C-Circulation check for a carotid or radial pulse if they don't have a pulse begin CPR
D-Deadly Bleeding treat any severe external bleeding

Treat any injuries found. If there is more than one injury or casualty treat the most severe, life-threatening problems first.

Secondary Survey

The secondary survey should be performed if medical help is delayed or if the casualty has multiple injuries. It invovles taking the history of the casualty, their vital signs and a head to toe exam.

History

The purpose of taking a casualty's history is to understand the circumstances leading up to the medical incident to better care for the casualty. There are five parts to the history of the casualty:

1. Symptoms-how does the casualty feel? (eg. nausea, pain)
2. Allergies-does the casualty have any allergies? (eg. penicillin)
3. Medications-has the casualty taken any medications in the past 24 hours?
4. Past Medical History-does the casualty have any chronic conditions?
5. Events Leading to the Incident-What was the casualty doing before the indicent?

Ideally, you should ask the casualty about their history; if they are unconcious then try to get as much information as you can from anyone who was nearby when the incident occured.