Head south from Cerulean City on Route 5 to reach the house in the center of the route. The old woman in the house will give you a Cleanse Tag, which will help keep wild Pokémon away from you.
You'll have to drop down from the top to get to this house. The old woman will give you a Cleanse Tag which, when equipped, will reduce the number of wild Pokémon encounters you face. After Route 5, cut west through Saffron City and Route 7 to get to Celadon City.