As soon as Ralph realizes that there are no adults, he starts to try to act in a responsible way. He starts to try to set up an organized "government." He starts to think about what they need to do to survive.
For example, Ralph decides that they need to find out how many other kids are on the island. When Piggy finds the conch, they use it to do so. In the resulting assembly, Ralph starts to try to organize their new "society."
However, he doesn't do this stuff right away. He goes swimming first -- until they find the conch.
As Lord of the Flies opens, Ralph is by himself on the beach and is soon joined by Piggy. Piggy convinces Ralph that there are no adults on the island. Ralph is content to behave as most twelve-year-old boys would. He does handstands, takes his clothes off, swims in the lagoon, and lies on the beach and covers himself partially with sand. He is enjoying the environment of the "coral island"; he is probably dreaming of the adventures a boy can have on a deserted island.
Piggy prods him a couple of times to try to begin finding the other boys, but Ralph has no interest. Ralph discovers the conch, but Piggy is the one who suggests it can be used to call the others, although he credits Ralph for the idea even as he suggests it. When the other boys appear, Ralph is the first to suggest that the boys elect a chief. Ralph wins the "election," although he has really displayed no leadership abilities or special intelligence. The qualities that work in his favor are "his size, and attractive appearance, and most obscurely, yet most powerfully, there was the conch." Ralph becomes leader by a quirk of fate more than anything else. He is not naturally inclined to leadership; he likes to have fun and goof off just like other boys. Piggy is the one who possesses the most intelligence, but his appearance disqualifies him from leadership. Jack is "the most obvious leader." As Shakespeare said, "Some are born great, some achieve greatness, and some have greatness thrust upon them." In Ralph's case, he does nothing at first to seek his high position; rather, greatness is thrust upon him.