In Chapter 1, Ralph and Piggy look around the beach, wandering around wondering what has happened to the rest of the boys. Piggy sees a large pink and cream colored conch shell and thinks that it would make a good trumpet.
The conch shell is one of the most important symbols in the book, it represents law and order. It is Piggy who comes up with the idea to use the conch shell to summon the boys together for a meeting.
At the meeting, Ralph takes the idea one step further, saying that since there were no adults on the island, they would have to look after themselves.
In Chapter Two, as the boys assemble for a meeting, Ralph says:
"We can't have everybody talking at once. We'll have to have 'Hands up' like at school." (Golding)
The conch becomes a symbol for authority among the boys.
The conch is the evident symbol of hierachy, consensus, and the established order. (At the antipode: the beastly head with flies swarming all around.) Ralph, along with all the other boys, is suddenly propelled into a universe without fixed references or laws and has only his previous experience to go upon to bring chaos under control. He seizes the opportunity upon the discovery of the conch to 'sacramentalize' it by making it an emblem of order to evoke the boys' respect and cooperation. So the idea for using the conch to speak comes from within himself.
On a practical level, using the conch was a practical solution for convoking the boys scattered throughout the island since it could be heard from far away. It also obliged the boys to voice their opinions one at a time and gave everyone a chance to be heard.