You need to have an empty pokemon space for this.
1)Go to goldenrod. go up and into the gateway.
2)talk to the guy on the left and he'll ask u to deliver a pokemon to his friend.
3)say yes. now go to violet and go right, through the gateway, and talk to the chubby dude next to the bitter berry tree.
4)give him the delivered pokemon(named kenya)and he'll read the mail.
5)he'll thank u by giving u TM50 Nightmare, which vuts 1/4 of the opponent's health when they're asleep.
6)go back to the guy who gave u kenya and he'll thank u by giving u an HP UP.