How do you get po mossdeep city on Pokemon ruby?

you have to

not

you have to get the red orb then you have to go to team maga HQ you have to get to the sub then battle and don't forget the master ball to cacth groudon later in the game

If you mean steven, the champion, he lives in the house in the upper left corner of the island.

start at lily cove and keep surfing to the east til u find and island