Touch the victim and ask them "Are you OK?" If there's no response,
call 911, or ask someone else to call.
Check the victim's breathing. Sometimes a person in cardiac arrest will make grunting, gasping, or snoring sounds. Don't confuse this with normal breathing.
Open
the victim's airway by tilting his/her chin gently with one hand, while
pushing back on his/her forehead with the other hand. If you suspect a neck
injury, put your fingers behind the jawbone just below the ear and push
the jaw forward to open the victim's mouth. 
Pinch the person's nose, take a deep breath, and blow into the victim's mouth. Pause and repeat.
Find the xiphoid process at the end of the sternum, and place the heel
of one hand two finger widths above this point. Place your other hand
on top and interlace your fingers. Now straighten your arms, lock your
elbow, and position your shoulders directly over your hands. Push down
on the victim's chest 1.5 to 2 inches for an adult. Do four sets of 15
compressions and 2 breaths over a 1-minute period. Do not stop to check
for a pulse.