On arrival at the scene, think (SHAPE)

Safety and Protection

You may need to take safety precautions when dealing with a casualty to protect yourself from cross contamination, e.g. from bodily fluids such as blood, vomit, urine or faeces. Precautions could include wearing gloves or a breathing mask.

Hazards

E.g. The casualty may still be holding on to an electrical appliance and you will need to make sure that the power is turned off before you touch the casualty.

Assess

Assess the situation. Make sure that neither the casualty nor yourself are in danger

Prioritise

In a situation with more than 1 casualty you must attend the casualty with the most serious or life threatening injury first.

Environment

e.g. The casualty could be in the middle of a busy road and so firstly you would need to ensure that somebody is controlling the traffic.