Instructions
Fly to Solaceon Town and head north onto Route 210. You will eventually find a group of Psyducks blocking the path. The Psyducks are all suffering severe headaches and won't move until they are cured. Use a Secret Potion and they will instantly be healed. Cynthia will arrive and give you the Old Charm. She will ask you to give the item to her grandmother who lives in Celestic Town.
Make your way to the Celestic Town Ruins and defeat the Galactic Grunt that guards the entrance. He will use a level 25 Beautifly and a level 27 Croagunk in "Pokemon Diamond" and "Pokemon Pearl." He will own a Golbat and Houndour at level 30 and 32 respectively in "Pokemon Platinum." Make sure you have suitable Pokemon in your party to bring into battle. Both Beautifly and Golbat are weak against electric-type attacks whilst Croagunk will faint after a few psychic moves and Houndour won't stand a chance against a water-type Pokemon.
Stand by the entrance to the ruins and an elderly woman will arrive. She will engage you in conversation and reveal herself to be Cynthia's grandmother. Give her the Old Charm and proceed into the Celestic Town Ruins. Examine the ancient sketches on the northernmost wall. Cynthia's grandmother will follow and talk to you once more. She will give you HM03 -- Surf, which allows you to ride on water, after explaining the artwork on the wall.
Exit the ruins. You can now leave Celestic Town and continue exploring the Sinnoh region if you are playing "Pokemon Diamond" or "Pokemon Pearl." However, Cyrus, the leader of Team Galactic, will challenge you to a Pokemon battle if you are playing "Pokemon Platinum." Cyrus will use three dark-type Pokemon ranging from level 34 to 36. Use a fighting-type or bug-type to easily defeat Cyrus and leave the Celestic Town Ruins.