On a whim, I did EMT-B training a few years ago. Part of that course is a responder-level CPR cert. Through muscle memory, one of my fellow students jumped on scene during the evaluation, and the first thing she did was instruct a bystander to "Call 911!"

To which the evaluator responded, "You are 911"

It’s essentially 112 (at least as a secondary number) almost all around the world thanks to ITU

That's what they taught us in Boy Scouts. Very first step is to get help.

Have someone call 911 (or whatever number in your country) immediately. If nobody is nearby start yelling.

Only one time in my life have I had to perform CPR (was already too late) but it is amazing how much of what I learned when I was 11 years old stuck with me.

Get help, check airway for obstructions...etc. Most importantly...take a CPR class sometime in your life and do what they say. The "ABC" of airway, breathing, circulation they taught us may not be current advice.

Sidenote: Don't be too hard on yourself if you don't save a life...even the pros say unless the patient is young/healthy you likely won't.

Edit: The feeling when the pros actually show up (in the case where I was doing CPR within minutes) is one of the biggest reliefs I have had in my life. Even knowing CPR...you feel helpless until they show up. And once they do, ask immediately if they need you. If not...get out of the way.

Step -1: touch somebody and tell them to call 911.