Mr. Stone asks you to take a letter to Steven in Dewford Town, so go to Mr. Briney's cottage in Route 103 and ask him to take you to Dewford Town by boat.

Get Some Items

Go into the house next to the dock and talk to the person at the table to get a Silk Scarf.

Talk to the guy south of the dock and say Yes to get an Old Rod. You can use the Old Rod while facing the water to encounter different types of pokémon. Using the Old Rod in Dewford Town you can find Tentacool and Magikarp.

Talk to the person near the Pokémon Center to change the Trendy Phrase. This affects where you can find a certain pokémon later.

Challenge the Gym

You should go to the Dewford Gym. The trainers here use Fighting pokémon. They are weak to Flying, Ghost, and Psychic moves. You should avoid bringing Normal, Rock, and Dark pokémon. Wingull and Taillow are good choices. You can also go northwest to Granite Cave and catch more pokémon there or train your current pokémon. You can talk to Mr. Briney if you want to go back to Route 103 and explore the areas that you have already been to.

Go to Granite Cave

You still have to deliver the letter to Steven. He is in Granite Cave to the northwest. When you are ready, go northwest to Granite Cave.