What rule does Ralph lay down at the first assembly and what does the frightened boy tell the group, how does it affect him?
I think that you are thinking of the second meeting, and not the first one where they elect Ralph as chief.
At this second meeting, the main rule that Ralph lays down at the is that the person who is holding the conch is the one who gets to speak. All others must listen to that person.
A little later on in the meeting, the frightened boy tells the group that there is a beast on the island. At first he says it is a snake, and then he says it is a "beastie." Of course it frightens him and it ends up frightening most of the kids.