Together, Piggy and Ralph discover the conch shell. Piggy is the one to explain how it is used, and this is indeed what Ralph uses to call attention to where he is on the island. As he uses the conch, the boys come from all directions. This summons is what begins their "democracy" on the island. However, more importantly, the conch is a symbol not only in this first part of the novel, but up until it is shattered. This link will explain the symbolism of the conch shell and its importance throughout the novel. Notice also that as the story continues on and Jack pulls away from Ralph's group of boys, the shell becomes more fragile each day. This is also symbolic of thier control (or lack there of).
He uses a conch shell. It's symbolic of the boys attempt to maintain the order and control of a society they're accustomed to. When Roger destroys it, the result is chaos and anarchy.