After going through the beach of Route 109, you will reach Slateport City, where you have to deliver the Devon Parts to Captain Stern.
In the southwest part of town, there is a market where you can get items if you know the move Secret Power, so be sure to come back when you get that move.
In the southwest corner of that market, you can find an Alakazite by checking the sparkles on the ground.
In the market, you can also buy incense, vitamins, and TMs.
Pokémon Fan Club
In the Pokémon Fan Club, you will find a woman who will style a Furfrou's hair.
There is also a woman there who will give you a Soothe Bell if your lead pokémon has high happiness.
The Name Rater's house is north of the Pokémon Fan Club. He can change your pokémon's nickname (unless someone else was that pokémon's original trainer).
Go into the Contest Hall and talk to the sailor in the northwest corner to get TM41 Torment.
You won't be able to participate in contests until a little bit later in the game.
Give the Devon Parts to Captain Stern
Go into Stern's Shipyard. It's an orange building near the southeast of Slateport. You will find Dock there. Dock is in charge of designing the ferry. He asks you to go to the Oceanic Museum to give Captain Stern the parts. Before you go, you can go upstairs to check out the vending machine, which sells Fresh Water, Soda Pop, and Lemonade.
The building north of the Shipyard is the Oceanic Museum. Go up and pay the entry fee of 50 Pokédollars.
Talk to the Grunt in the southeast corner of the ground floor of the museum to receive TM46 Thief.
Go upstairs. (Both stairs go to the same place.) Talk to the old man in the southeast corner. He is Stern. As soon as you try to give Stern the parts, some Grunts run up to try to take them.
The first Grunt has Carvanha. The second has Zubat and Carvanha.
After you defeat them, the leader of Team Magma/Aqua comes up and says a few words, then leaves with the Grunts.
After that, you can go downstairs. There are visitors in the museum now. There will be more visitors upstairs if you go up there again.
Otherwise, just leave the museum.
Get Scouted for Contests
Try to go north through the exit of Slateport, and you'll hear some Ace Trainers say that Lisia and Ali are doing a shoot. You will go over to the Contest Hall, and Lisia will choose you as the next trainer to participate in contests. She will give you the Contest Pass, which lets you participate in contests, and the Pokéblock Kit, which lets you turn berries into Pokéblock candy for your pokémon.
If you feel like it, go into the Contest Hall and walk up to the front desk. Lisia will take you to the dressing room and give you the Contest Costume.
Now you can go enter a contest. For more information, read the Contest Guide.
After your first contest, a woman in the lobby will send Cosplay Pikachu to your PC Box. You will be the original trainer of this Pikachu. Read more about Cosplay Pikachu.
If you participate in a contest, be sure to talk to the people in the lobby of the contest hall afterward to receive gifts from them.
After you are done with Contests, go north to reach Route 110.