For a first timer, you are most likelly going to be doing a startle scare of some sort (were you jump out and scare) or a victim for another actor. Just remember, a scare lasts about 5 seconds. After that you are jsut on their nerves. You see some "actor" hawking the one girl in the group who is terrified all the way out of the room and down the hall but everybody else is annoyed. Most likely the hawk screwed up somebody elses scare in the process. When I'm working the kids around me learn real quick "you've got your space to scare them and i've got mine. Unless you want to get scolded in a hurry you better stay out of my space. You screw up MY show and I'm going to get pissed. And it probably will only happen once."
A good scare- Get in hard, loud, and fast- do your thing- and get back out- preferably out of sight so they know its over and move on. Experience will guide you.