Once you beat the other four Gym Leaders (naturally), you must head back to Petalburg City where you have to fight your dad, Norman. He specializes in normal-type Pokemon and uses two Slakings and a Vigoroth against you.
Rustboro City is north past Petalburg Woods. From Oldale Town go west to Petalburg City and continue west until you reach the ocean. From there go north to Petalburg Woods and continue north until you reach Rustboro City. Oldale Town is just north from Littleroot Town.
You must have a friend that can give u a Pokemon that already learned the attack name surf , because there is a pond in the south of petalburg city to meet the gym leader Norman in the cave, and he can give u the key for the gym name petalburg city gym.