Who is the first boy to die? Who announced it?

The first person to die is the boy with the mulberry-colored birthmark on his face, who first spoke of the Beast during the assembly. 

The boy's death is not narrated, nor is his body found; he is simply never seen again, leaving everyone to assume that he died. His death is partially representative of the loss of innocence on the island, due to the fact that he introduced the Beast, but died from the boy's...

The first person to die is the boy with the mulberry-colored birthmark on his face, who first spoke of the Beast during the assembly. 

The boy's death is not narrated, nor is his body found; he is simply never seen again, leaving everyone to assume that he died. His death is partially representative of the loss of innocence on the island, due to the fact that he introduced the Beast, but died from the boy's negligence. 

Following the assembly, the boys decide to build a fire on top of the mountain to facilitate their rescue. Being rushed and careless about it, they allow the fire to grow too large, and some of the sparks it throws off ignite the dry wood on a part of the mountain where some of the littluns had gotten distracted and began looking for fruit instead of helping with the fire. Suddenly the place where the littluns were is on fire, and Piggy is attempting to argue some sense back into everyone by trying to account for all the boys, and stating that he doesn't see the birthmarked boy. This seems to wake the others up, as they all realize this boy is unaccounted for, and is probably dead.