If the instructor stands in front of your rib or behind you, don’t worry! In the first case it is to observe if the dominant eye is aligned just above the rib, in the second to check your stance. The following tips are geared towards Olympic Trap, but can be adapted for other disciplines (see end of article).
It all depends on your level as a shooter and your natural talent. If you have never shot clay pigeon before, I recommend that you first go to the nearest shooting range to try it out. There might be an instructor and maybe they could even lend you a shotgun for your first attempt.