Jim is both the narrator and the central character of the book, which means that we follow his personal view of events from his encounter with Billy Bones at the Admiral Benbow Inn to his departure from Treasure Island on the Hispaniola. The only exception is Chapters 16 through 18, when Doctor Livesey takes over as narrator to fill us in on events that Jim isn't present to witness. But Doctor Livesey and Jim have surprisingly similar types of observations, so it doesn't feel like a jolt to move from one narrator to another. For more on both Jim and Doctor Livesey as first person narrators, check out "Characters."
We're just going to add one more note about narrative voice in this book: we keep mentioning that this is an adventure novel. The great thing about a first-person narrator is that it's pretty easy for the reader to identify with an "I" telling a story, because we all think of ourselves as "I." As we read Jim's experiences aboard the Hispaniola and on Treasure Island, the first-person narrator makes it even easier for us to put ourselves in Jim's place and to imagine what we would do if we were facing down Long John Silver.