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.