The movie adaptation of William Golding's Lord of the Flies differs from the novel in many ways. For example, the boys in the book are British boarding-school boys while the boys in the movie are American schoolboys. In the book, the boys are somehow deposited on the island when the plane they are on crashes; in the movie, the boys are on a life boat after their plane crashes and then make their way to the island.
In terms of leadership, the book makes it clear that Ralph was elected leader of the island based on his appearance, demeanor, and position as one of the oldest boys on the island.
But there was a stillness about Ralph as he sat that marked him out: there was his size, and attractive appearance; and most obscurely, yet most powerfully, there was the conch.
Jack is upset that he was not elected as chief, and he walks away with his own choir boys who become his hunters. The two of them, Jack and Ralph, are constantly doing battle (mostly figuratively) for control of the boys and therefore the island.
In the movie, though, Ralph is the clear leader of the island at the beginning and is a much more effective leader, at least for a while, in the movie than the book. For example, Ralph is the one in the movie who makes fire with Piggy's glasses, and he does not join in the killing of Simon. Jack eventually leaves the tribe and just asks who wants to go with him. Some do. By the end of the movie, however, Jack does have his tribe of savages.
All of these things are good reasons, by the way, why one should not judge a book by its movie.