Step one: you are going to make sure the scene is safe 

Make sure that nothing that is unsafe around and if there is something unsafe wait for help

Step two: call for help or have somebody that is with or near you call for help

If nobody is near you call for help quickly your going to want to call 911

Step three: check and see if you can get a AED 

a AED is something that you can put on a persons chest and lets out a shock

Step four: check and see if the person is breathing

you are going to put you head near the persons face looking towards there feet if the chest rise and fall they are ok they are breathing if you feel air on your face then they are breathing