Used to train CPR back in the day. Some things of note:
CPR is used to keep someone alive until real medical care arrives at the scene. You are not bringing someone back to life. You need to call 911 (or tell someone specifically to call 911 while you do CPR) and then do CPR until then. But you won't revive them, you're basically just pumping their heart for them.
If this is in a public place, there might be an AED nearby. Use it. It takes no professional training whatsoever to use, in fact it's about as idiot-proof as possible. These things work wonders and elementary school children have been able to use them.
If you do not have an AED and must do chest compressions, you will break ribs. It will be a nasty affair. It will also be the most tiring thing you can do. 30 seconds (50 compressions) will wear you the fuck out. If there is someone else on the scene who knows CPR, please have them cycle in because you could be going several minutes doing this.