Before attempting CPR, be sure to call 911!
CPR is best done by someone trained in the technique, but it is as simple as ABC. 

A - Airway
First, open the airway by gently lifting the chin with one hand and pushing down on the forehead with the other hand to tilt the head back. 

B - Breathing
Once the airway is open, lean over and put your ear close to the victim's mouth to determine whether the person is breathing. If the victim is breathing, roll him onto his side to wait for help. If there are no signs of breathing, and opening the airway does not trigger breathing, "rescue breathing" is necessary.

The best way to give rescue breathing is by using the mouth-to-mouth technique.

• Using the thumb and forefinger of the hand that is on the victim’s forehead, pinch the person’s nose shut. Be sure to keep the heel of your hand in place so the person’s head remains tilted. Keep your other hand under the person’s chin, lifting up. As you keep an air-tight seal with your mouth on the victim’s mouth, immediately give two full breaths.

• After giving two full breaths, find the person’s carotid artery pulse to see if the heart is still beating.

• Take the hand that is lifting the chin and find the person’s Adam’s apple (voice box). Slide the tips of your fingers down the groove beside the Adam’s apple and feel for the pulse.
  
  If you cannot find the pulse, besides providing rescue breathing, you will have to give chest compressions.

C - Circulation
Immediately give two full breaths, then find the carotid artery pulse in the neck (near the Adam's apple) to see if the heart is still beating. If you can't find the pulse, you will have to attempt artificial circulation along with rescue breathing.

• Kneel beside the victim’s chest. With the middle and index fingers of your hand nearest the person’s legs, find the notch where the bottom rims of the two halves of the rib cages meet in the middle of the chest. Now put the heel of one hand on the sternum (breastbone) next to the fingers that found the notch. Put your other hand on top of the hand that is in position. Be sure to keep your fingers up off the chest wall. It may be easier to do this if you interlock your fingers.
• Bring your shoulders directly over the victim’s sternum and press down, keeping your arms straight. If the victim is an adult, depress the sternum one and one half to two inches. Then completely relax the pressure on the sternum, letting the chest rise to its normal position but not removing the heel of your hand from the sternum. Relaxation and compression should take equal amounts of time.
• If you must give both rescue breathing and external chest compressions, the proper rate is 15 chest compressions to two breaths. You must compress at a rate of 80 to 100 times per minute.