Even if you don't know CPR, the American Heart Association has some advice for you, in the form of this one minute video. In short, if an adult collapses, call 911 and begin chest compression—don't wait or try to wonder how to do it correctly; quick action can save a life.
Of course, this video is absolutely no substitute for learning proper CPR and getting yourself certified—I definitely recommend doing it, and it only takes a day-long class to do. It's one of the many basic life-saving skills everyone should know. Still, if you don't know CPR and don't know what to do when someone suddenly collapses, this can help. The technique is called "hands-only" CPR, and according to this American Heart Association fact sheet, it can triple a heart-attack victim's chances of survival. If you absolutely need a rhythm to do the chest compressions to, the AHA suggests the disco classic “Stayin’ Alive,” which, irony aside, has approximately 100 beats per minute—right on time for how quickly you should compress.
This One-Minute Video Will Turn You into a Life-Saving Superhero | Greatist