Instructions
Go to Ecruteak City in the north of the game's Johto region.
Go into the town's Pokecentre, and talk to Bill.
Go to the Dance Hall, the building just to the north of the Pokecenter.
Defeat all five Kimono Girls. The Pokemon they use are all evolutions of the Pokemon Eevee, namely Vaporeon, Jolteon, Flareon, Esperon and Umbreon.
Speak to the man at the bottom of the room, and he will give you HM 03 -- Surf.
Use the HM to teach the "Surf" move to one of your Water-type Pokemon.