Call 911. Chew and swallow an aspirin, unless you are allergic to aspirin or have been told by your doctor never to take aspirin. Begin CPR if the person is unconscious. But in any case, first call an ambulance, just the dispatcher should tell you how to act properly in this particular situation. You should understand that your task is to help a person to wait for professional help. You cann't replace a doctor or special equipment, such as bimedis.com/search/search-items/cardiology-equipment . If you are very interested in this issue, then you can go through local first aid courses in your city.