Ralph is the de-facto leader of the band of boys on the island because he is the one who blows the conch first. A somewhat selfish boy, he does not realize his power or responsibility. He does not take advice, especially Piggy’s, when he should.
When Ralph blows the conch, it is the first time everyone is brought together.
Their heads clustered above the trunks in the green shade; heads brown, fair, black, chestnut, sandy, mouse-colored; heads muttering, whispering, heads full of eyes that watched Ralph and speculated. Something was being done. (ch 1, p. 23)
Ralph brings civilization to the island by gathering the boys. He has power through the conch, and the conch seems to imbue him with power too.
When the boys decide to vote for a leader, Ralph is a logical choice.
"Him with the shell.”
“Let him be chief with the trumpet-thing.”
Ralph raised a hand for silence. (ch 1, p. 29)
Ralph is glad to accept the role. He even takes charge at first by assigning the choir to Jack and putting him in charge, and telling the boys to explore to determine if they are on an island. He leads the team of explorers and then draws everyone together to talk about the need for meat. He even soothes the littleuns, explaining that there can be no beast.
Yet Ralph is interested in power for power’s sake, not in order to help the boys survive. He ignores Piggy, even though he repeatedly has intelligent suggestions. Most of the time Ralph does not try to take advice.
Ralph turned and smiled involuntarily. Piggy was a bore; his fat, his ass-mar and his matter-of-fact ideas were dull, but there was always a little pleasure to be got out of pulling his leg, even if one did it by accident. (ch 4, p. 91)
In the end, Ralph’s refusal to actually try to set up a regular governance structure and keep order leads to the deaths of both Piggy and Simon as the tribe splits in two and descends into anarchy.