This is a key question for Golding's novel. After the plane crash, the boys are alone and unsupervised on the island, which means that all external obligations and responsibilities are absent. How the boys will act is what Golding explores.
In the opening pages, Ralph and Piggy meet, and Ralph states, with enthusiasm: "No grownups!" (8). Adults represent authority and the social structure, and the boys are initially excited, if anxious, that they are in charge of themselves. Ralph and Piggy, along with Simon, who represents innocence and purity, are the most moral characters in the book and the ones who both do no succumb to the violence and anarchy of the others and attempt to create a mini-civilization on the island, replete with jobs, rules, and a structure. They make a fire (symbolizing civilization and wisdom) to cook and to signal to ships, they build structures, and they use the conch shell to bring some order to the group. The shattering of the conch later in the book brings an end to this fragile order.
On the other side is Jack, who quickly sheds the vestiges of civilization and morality and embraces his violent, primal side. His character is clear from the start when he insults Piggy: "Shut up, Fatty" (21). He and the other boys, especially Roger, become more savage as the book progresses (or regresses, if you will), turning into hunters, ignoring the rules that have been set up and, finally, murdering Piggy by dropping a rock on him. When the book ends, the island is on fire and they're all trying to kill Ralph. The center does not hold.
Note: I'm using the Perigee paperback.