Final answer:
The very first thing to do when someone suddenly collapses is to check for responsiveness and call for emergency medical help.
Step-by-step explanation:
If you see someone suddenly collapse, the very first thing you should do is check for responsiveness. Tap the person gently and ask if they are okay. If there is no response, call for emergency medical help immediately by dialing the emergency number in your country (such as 911 in the United States). While waiting for medical help to arrive, you can begin performing CPR if you are trained and confident in doing so.