4 steps

Early access - The patient must be tended to quickly. If he does not receive any attention or aid within a few minutes (eg heart attack at home alone), the chances of survival are slim

CPR - Someone must be able to perform cardio-pulmonary resuscitation, with chest compressions on the lower half of the sternum, adequate depth of compression is about 4 to 6cm. Don't worry too much if you hear cracking of the ribs. 30 compressions to 2 breaths (aka the kiss of life). Just ensure that you open the airway before blowing.

Defibrilation - Hopefully, you have already tasked someone else to get you an Automated External Defibrillator while you were doing CPR. Proper and safe use of the AED does boost the survival rate greatly.

Advanced care - Of course, it is too much to hope that bystander intervention is enough to ensure survival. Someone should have called the ambulance or EMS, so that the patient can be brought to hospital for advanced cardiac life support.