Beyond Route 102, you find Petalburg City, where your Dad, Norman, is the gym leader of the Petalburg Gym.
Help Wally Catch a Pokémon
Go up to the Petalburg Gym in the north part of the town. You will see your Dad, Norman, talking to a pokémon trainer. Afterward, Norman invites you into the gym. After you talk a little bit, Wally comes in and says that he will be staying with his family in Verdanturf Town. Wally asks if he can have a pokémon, and Norman lets him borrow his Zigzagoon. Norman asks you to help Wally catch a pokémon. You will go together to Route 102 and Wally will encounter a Ralts. He will automatically battle and catch the Ralts (you don't need to do anything).
Get a PokéNav Upgrade
Afterward, you both go back to the gym. Wally will add the PlayNav to your PokéNav Plus. This adds the PSS, Pokémon-Amie, and Super Training. The PSS allows you to connect to the Internet and interact with other players. Pokémon-Amie lets you become closer friends with your pokémon. Super Training allows you to do EV Training, which boosts your pokémon's stats.
If you want to receive notifications, be sure to open up the PSS, connect to the Internet, and tap the icon in the top middle, then choose Holo Caster. After that, tap Activate SpotPass if you want to get notifications about online competitions and pokémon Mystery Gift distributions.
Norman says to go to Rustboro City and challenge Roxanne. After you have gotten some Gym Badges, you can go back to Petalburg and challenge Norman.
When you are done exploring Petalburg City, go west to reach Route 104 South.