Personally, the greatest thing you could do now is to take a CPR course offered by the American Red Cross ( https://www.redcross.org/take-a-class/cpr ) or from the American Heart Association (http://cpr.heart.org/AHAECC/CPRAndECC/FindACourse/UCM_473162_Find-A-Course.jsp ). These are excellent, state of the art courses that will provide the tools needed to provide the best care possible out-side-of-a-hospital for an individual suffering from a heart attack.
Finally, in general, until help arrives: place the person in a safe comfortable place, work on getting them to calm down and to breath comfortable and easily. Loosen any tight restrictive clothing and if they have medication for their heart that they are supposed to take (generally a small nitroglycerin pill that goes under their tongue) help them take that medication. They're likley to vomit so only a sip of water at best and otherwise nothing to eat or drink. Thats about the best you can do without CPR training, which is the next thing you should do now to increase the odds of a good outcome, if necessary, in the future.
You asked what to do in a “cardiac arrest.” This implies that the patient is already unconscious and without a palpable pulse. In that case, start CPR, call 911, and ask for a defibrillator (if available).
Cardiac disease can be very tricky, even for a Board Certified ER Dr., again, because not everyone has textbook symptoms. I have personally found 2 patients over my >20 year career who complained of a toothache, but something about them wasn't right. I asked them cardiac questions, we did an EKG, and they were both in the midst of having a heart attack. So I never let my guard down when it comes to this diagnosis. Hope that helps.
-Check for response
-Check neck pulse and breathing at the same time.
-If no pulse, begin CPR (chest compression = push fast and push hard, center of the chest). Don't stop until EMS gets there.