you get by climing up the ladder in seafome island and its behind the legendy
^ not sure if the above is applicable since I haven't played for a while, but you can get one early on by not evolving Charmander, Squirtle and Bulbasaur and show them to the Pokemon Fan Club Members in Vermillion City, after that, talk to the Chairman of the fan club and he'll give you one.
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an exp. share works on any pokemon your welcome
All you have to do is just put a pokemon (preferrably a level 100, or at least your highest leveled pokemon) in the front of your party, and give the pokemon you want to train up the exp. share. Then it will get a share of the exp. points. The reason you would want a level 100 in front is because then the pokemon holding the experience share will get 100% of the exp.
After you catch 50 Pokemon, have Prof. Oak look at your pokedex. He will send an aid with the Exp. Share
go into one of the houses and talk to the man show him your basic Pokemon and he'll give you an exp. share.
When you get a red scale from gyardos, Mr. Pokemon will want it.Go to his house and give it to him and he will give you the exp. share.