You can prepare in advance by making sure that you know the emergency service number in your country - in the US it is 911, in the UK it's 999 here in France it's 117 - so it's different for every country. Find yours and put it into your land-line and mobile phones on speed dial.
If you think you are having a heart attack - grab your phone, press the correct speed dial and tell the person who answers where you are. Keep the line open until help arrives.
Chew an aspirin and call 911 immediatly if you are able to. If not, call for help as loud as you can until someone hears you and can call for help.
My father-in-law had a severe heart attack a few years back and the paramedics that arrived said that chewing the aspirin is what most likely saved his life.
keep isosorbide-5-di nitrate tablet under your tongue