Instructions
Go to Celedon City. Playing the game will take you there naturally; you cannot access Celedon City early.
Enter the Celedon City Gym. It has a sign in front stating its name. It is the largest building in the city.
Walk up to Erica and press "A" to initiate battle.
Fight Erica just like all battles you had before this point. Erica will use Victreebell, Tangela and Vileplume, all of which are Grass-type Pokemon. Fire-type Pokemon do double damage against Grass-type Pokemon. Avoid Water-type Pokemon, as they are weak to Grass-types. When you defeat all of her Pokemon, she will give you the Celedon City Badge.