Train in a manner that's appropriate for a youth your age. Remember that overextending your body while you're still growing can do damage that will give you trouble in the future. Familiarize yourself with proper form for your sport and realize that you won't be able to do some of the things that adult athletes do until your body fully matures.
Keep yourself in good physical shape, even in the off-season. You'll be that much further ahead when the season starts than others who spent the off-season on the couch. Even when you are off, you need to practice consistently to stay ahead of the game.
Learn about good sportsmanship. As an athlete, you need to be able to both win and lose gracefully. Taunting the losing team, or refusing to congratulate the winning team are not ways to become a great youth athlete.
Listen to your coaches and your parents. Show them the respect that they deserve. They both have an interest in helping you become a great youth athlete. They are going to be focused not just on winning, but building your character through athletics.
Model yourself after athletes who deserve to be emulated. Those who will do anything to win, use drugs to enhance their performance or are more focused on themselves than the team as a whole are not ones you want to pattern yourself after.