Unlike previous Pokemon Games, you'll receive the Exp Share quite early in this one, at the Trainer's School to be exact. Since you will still be New to your journey, this item becomes all the more valuable to your success. It will automatically be on after obtaining it, and if you want your Pokemon to grow as quickly as possible, you'll never turn it off. With the help of this item, most, if not all of your Pokemon should be around Level 60 by the Time you take on the Elite Four!