Like most of Ralph's other good ideas, it actually comes from Piggy. Ralph fishes it out of the water in the first chapter, towards the start, and Piggy immediately is the one who knows what it is and that it can be blown to make a sound:
Ralph had stopped smiling and was pointing into the lagoon. Something creamy lay among the ferny weeds.
“No. A shell.’ ’ Suddenly Piggy was a-bubble with decorous excitement.
“S’right. It’s a shell! I seen one like that before. On someone’s back wall. A conch he called it. He used to blow it and then his mum would
come. It’s ever so valuable—”
Ralph is fascinated by it without any real sense of how it might be used. And then Piggy has the really good idea, and immediately credits Ralph with having had it. As usual, Ralph passively accepts the credit:
Piggy paused for breath and stroked the glistening thing that lay in Ralph’s hands.
Ralph looked up.
“We can use this to call the others. Have a meeting. They’ll come when they hear us—”
He beamed at Ralph.
“That was what you meant, didn’t you? That’s why you got the conch out of the water?”
It is Piggy (representative of intellectualism in the novel) who gives Ralph the idea to make the conch into a tool of civilization.
Piggy says, "It's a shell. I seen one like that before. On someone's back wall. a conch he called it. He used to blow it and then his mum would come. It's ever so valuable--" (11).
The conch becomes a speaking stick and a trumpet of sorts, rallying the boys together so that Ralph and Jack can address the troops.
Piggy tells him about how he blew one before