What should I do if someone is having a heart attack besides call 911? What should I do if I'm having a heart attack?

Calm, aspirin. 911 most important. If available have them chew (works faster) 1-2 aspirin. Try to keep them comfortable and calm. If they become unresponsive and start cpr.
Call 911. You or someone should call 911, it is important that you reach a treatment center where there are facilities to treat heart attack including clot buster to save the heart muscle.