Go to the S.S. Anne in Vermilion City
Get the S.S. Ticket from Bill in Cerulean City
Board the S.S. Anne
Defeat your rival on the ship
Enter the captain’s cabin
Talk to the captain and rub his back
Receive the Exp. Share from the president of the Pokémon Fan Club in Vermilion City after showing him at least 50 Pokémon in the Pokédex