You and a friend are walking in a park when you see a man collapse and fall to the ground. after gently shaking the man and asking "are you ok? " there is no response, and he is not breathing. your friend calls 911. what should you do?

Look for a pulse and do not leave his side until help arrives. 

Check for a pulse. if there is a pulse tilt his head back to open up the airways then check if there is anything caught in his throat. for example if he choked. slowly start administering mouth to mouth. If there is something stuck after every to sets of mouth to mouth see if you can take it out. if there isn’t something stuck check to see if you hear breathing every 3 sets

Very minute and important evidence needs to be collected in order together detail and figure out what had happened