Just a Few Simple Steps (Could SAVE a Life!)

1. Activate EMS (Emergency Medical Services)
Check the injured person for responsiveness.
If the person is not responsive and not breathing or not breathing normally.
Call "YOUR LOCAL EMS", USA 911 UK 999 Europe 112 etc. and return to the injured person. If possible bring the phone close to the person and place on speaker mode. In most locations the emergency dispatcher may be able to assist you with CPR instructions.
Important Point - DO NOT WASTE TIME LOOKING FOR A PULSE! If no signs of response begin compressions.
2. Begin Chest Compressions.

If the injured person is still not breathing normally, coughing or moving, begin chest compressions. Push down in the center of the chest 2-2.4 (4 to 5cm) inches 30 times. Pump hard and fast at the rate of 100-120/minute, twice as fast, than once per second.
3. Perfom EAR Expired Air Resuscitation
(Only if trained and comfortable with skill)
Tilt the head back and lift the chin. Pinch nose and cover the mouth with yours and blow until you see the chest rise. Give 2 breaths. Each breath should take 1 second.
More details about CPR...
CPR involves chest compressions for adults between 5 cm (2.0 in) and 6 cm (2.4 in) deep and at a rate of at least 100 to 120 per minute.
The rescuer may also provide artificial ventilation by either exhaling air into the subject's mouth or nose (mouth-to-mouth resuscitation "EAR") or using a device that pushes air into the subject's lungs (mechanical ventilation "BVM Bag Valve Mask").
Note the mouth to mouth has been dropped in many Countries, however if you have a pocket mask or BVM then this may be effective in helping recovery...
Current recommendations place emphasis on early and high-quality chest compressions over artificial ventilation; a simplified CPR method involving chest compressions only is recommended for untrained rescuers.
In children, however, only doing compressions may result in worse outcomes. Chest compression to breathing ratios is set at 30 to 2 in adults.
CPR alone is unlikely to restart the heart. Its main purpose is to restore partial flow of oxygenated blood to the brain and heart. The objective is to delay tissue death and to extend the brief window of opportunity for a successful resuscitation without permanent brain damage. Use of supplemental oxygen in addition to CPR may be greatly beneficial...
See IHMP Oxygen First-aid.
Administration of an electric shock to the subject's heart, termed defibrillation, (Use of an AED is highly recommended), is usually needed in order to restore a viable or "perfusing" heart rhythm. Defibrillation is effective only for certain heart rhythms, namely ventricular fibrillation or pulseless ventricular tachycardia, rather than asystole or pulseless electrical activity. Early shock when appropriate is recommended.
CPR may succeed in inducing a heart rhythm that may be shockable. In general, CPR is continued until the person has a return of spontaneous circulation (ROSC) or is declared dead. ITDA/IHMP (Stephen Craig-Murray) Are full members of the UKRC 854697 and also the ERC The European Resuscitation Council (ERC) is the European Interdisciplinary Council for
Resuscitation Medicine and Emergency Medical Care.