In Eterna City, go to the building that starts the biker route. Talk to the dude in the white coat, with black hair and glasses. He'll say something like, "Oh hello! You probably don't remember me, but I am Professor Rowan's assistant and Lucas/Dawn's dad! He told me to give this to you." he hands you the exp. share. Hope this helps, good luck on your Pokemon journey. I got this from wiki and you can get more if you get second place in the lottery.
