Who first mentions the use of the shell?

The shell first appears in chapter 1. Piggy and Ralph are together when they notice the shell, and Piggy suggests its possible use.
The two boys are both at the beach when they see something shiny on the shore and then realize it is a shell. Piggy has actually seen this type of shell used before.

"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—"

It is Ralph, however, who embodies leadership by blowing it and having the others come. The authority he exudes is related to the shell:

. . . most obscurely, yet most powerfully, there was the conch. The being that had blown that, had sat waiting for them on the platform with the delicate thing balanced on his knees, was set apart.