Well, if you mean getting the exp.share as soon as you possibly can, then you could sail to slateport or go to route 116 (since you can get the exp. share bascially whenever throughout the game after you finish the beginning). If you pick the latter (make sure you have rock smash), go through the cave and towards the end to be stopped by two rocks. Smash them by using rock smash and the guy there will give you the HM - Strength. Continue right and down till you see an exit. Go through and you'll arrive in Verdanturf (which is just east of Mauville and NE of slateport).
Chat with our AI personalities
After delivering the letter to Steven, go to Rubusto or whatever city and talk to the guy who sent you to give the letter to him. He'll give the exp share to you.
after you have delivered the Devon Goods from Rustboro, go back and see the president!
If you started with the older generations, particularly the GameBoy games, then you may not be familiar with how ridiculously easy the game have become And therefore may not understand how to use an Exp. Share, since in the newer games all you have to do is turn it on and your entire team gets an xp boost. In Emerald, this is not the case. The Exp. Share is not a Key Item in Emerald. look in your inventory under Items, and scroll until you find the Exp. Share. Give it to a pokemon and then fight battles as you would any other game. Trainers give the most xp. Unlike the newer games’ Exp. Share, only the pokemon holding the Exp. Share will receive half the total xp.
give the exp share to a weak Pokemon when you use any Pokemon to fight exp goes to the fighting Pokemon and the 1 with the exp share.
put exp. share on it & defaet the elite four