Do you have to cock a revolver befor the first shoot?

Depends, A "Single Action" revolver, such as the Colt Peacemaker, carried in the old West, DID have to be cocked before each shot. A "Double Action" revolver, such as a current day Smith & Wesson, does not require that the hammer be cocked- it CAN be- or the trigger can be pulled, and the hammer will rise and fall.