Cardiopulmonary Resucitation is a lifesaving technique useful in many emergencies such as: heart attack or near drowing, in which a person's breathing or heartbeat has stopped.
Here is an advice from the American Heart Association:
If you are not trained in CPR, then provide hands-only CPR. Hands-only CPR consists of uninterrupted chest compressions of about 100 a minute until paramedics arrive.
If you are a well trained and confident in our ability, begin with chest compressions instead of first checking the airway and doing rescue breathing. Start CPR with 30 chest compressions before checking the airway and giving breaths.
Mayo Clinic Review
Before you start CPR check:
Is the person unconscious? Tap or shake the person's shoulder and ask "Are you OK?"
If the person does not respond call 911 and start giving CPR.
If an AED is immediately available, deliver one shock if instructed by the device then begin CPR.