First things first, call 911. If there is someone else there with you, assign them the task of calling 911 and establish "scene safety." You have to make sure that you, or anyone else, don't get injured while helping the subject. Then, as a non healthcare provider, approaching them and verify if they are conscious. Ask a few simple questions: are you ok, what's your name ..etc. If they are unresponsive, start chest compression (CPR) until medical help arrives.