What can you do if you witness someone have a heart attack and stop breathing?

Start CPR: A witnessed cardiac or respiratory arrest is an indication to begin CPR and rescue breathing, while waiting for additional help. See the american heart association's website.

CPR: If their heart is still beating, they would need rescue breathing. If their heart has also stopped, they would need cpr. Calling 911 is necessary and they can sometimes talk you through this intervention if you are not trained to do this. If you want this training, you could take a CPR class with the heart association (among other organizations).

Call 911: If you know CPR start it after calling 911. If there is an automatic defibrillator nearby send someone to get it or get it yourself. This will talk you through what to do. Even if you don't know cpr, doing chest compressions quickly (about 100 per minute) until help arrives may save someone's life. Better yet, learn CPR and take refresher courses every 2 years.