The Burning Bridge (Ranger's Apprentice, Book 2) by John Flanagan

Will is an orphan who knows almost nothing about his parents. All he has ever been told was that his father died a hero. The king took him and several other orphans into his castle were they were to await the choosing. A time when these orphans were given the chance to choose how they would spend the rest of their lives. Will was denied his greatest wish of going into battle school to follow in his father’s foot steps. A ranger named Halt had attended the choosing that day and gave the king a note about Will. Unable to get his mind off the note Will slipped into the kings work room that night and attempted to read the note. The only reason he failed to read the note was because as he reach for it, halts hand suddenly grabbed him. Halt took Will to the king and the king told halt to allow will to read the note. The note said that halt would accept will as an apprentice, and so began Will’s adventures as a ranger.