1. Beat Morty. This requires you to view the scene in the Burned Tower with the legendary beasts. 2. Fight the Team Rocket member in the Dance Theatre. After the fight, someone gives you the HM. 3. Teach it to a Pokémon and Surf away.
If you used a cheat device, you're screwed.
I love girls with leprosy...that way I can take some home with me.