Which would you do first if a person is not breathing?

First, make sure you and the person is in a safe area, make sure their won't be traffic nearby. But don't try to drag that person, as their spine may be hurt already.
we should try to see if that person is conscious or not by calling their names and tap on their shoulder.
At the same time, assign a specific person nearby to call the ambulance, never shout "someone call the ambulance" as everybody may assume somebody else is gonna call.
Only preform CPR if you are well trained and passed exams.