You arrive in Dewford Town looking for Steven, and Mr. Briney recommends that you ask the Dewford Gym leader where to find him.
This is a Fighting-type gym. Fighting-type pokémon are weak to Flying, Psychic, and Fairy moves. Don't bring Normal, Rock, Steel, Ice, or Dark-type pokémon, because they are weak to Fighting moves.
You can buy Fresh Waters from the vending machine inside the gym.
The first trainer that you encounter here is Battle Girl Laura, who has Meditite.
After you defeat her, step on the switch near her to turn on the lights, revealing a section of the gym that you couldn't see before. But when you step off of the switch, the lights turn off again, so be sure to memorize the path, then walk through it.
At the end of the path, you encounter Black Belt Hideki, who has a Machop.
After you defeat him, step on the switch, memorize the path, then go through.
At the end of this path, you encounter Battle Girl Tessa. She has Meditite and Machop.
Afterward, step on the light near her. Two sides of the hallway will be illuminated, but if you look closely, you will see that only the left path goes through to the area above the conveyor belt. Go through the left path and walk north onto the big switch. You'll light up Brawly's room.
Brawly has a Machop and a Makuhita. Both have the ability Guts, which boosts their attack power if they are burned, paralyzed, poisoned, or asleep, so you may wish to avoid inflicting those statuses on them. They both know the move Bulk Up, which boosts their Attack and Defense.
Brawly's Machop knows Seismic Toss, which does damage equal to the user's level (in this case, 14). The Machop also knows Karate Chop, which has a higher chance of doing critical hits.
Brawly's Makuhita knows Knock Off, which removes the target pokémon's held item. The item will go back into your bag. The move does more damage if the target pokémon is holding an item. Makuhita also knows Arm Thrust, which has a chance of hitting between 2 and 5 times.
Brawly has a Super Potion that he might use to heal up his pokémon.
After You Defeat Brawly
Brawly gives you the Knuckle Badge after you win. This causes traded pokémon up to level 30 to obey you. Brawly also gives you TM08 Bulk Up.
Brawly tells you that Steven rushed off to Granite Cave, so that is where you should go next.
In the first floor of the cave, you can find Geodude, Makuhita, Abra, and Zubat. Geodude is rare here, but it's very helpful in the next gym, so try to catch one if you can.
Go west from the entrance and talk to the Hiker to get TM70 Flash. You don't need to use Flash yet, so if you don't feel like teaching it to a pokémon now, you don't have to.
Enter the cave door north of the Hiker to find Steven. He is looking at a giant mural on the wall. After some dialogue, you give him the letter. In thanks, he gives you TM51 Steel Wing. Then he leaves the cave.
There is nothing else to explore here, so just leave for now.
Talk to Mr. Briney
Now that you have delivered the letter to Steven, go back and talk to Mr. Briney. He says that the Devon Parts need to be delivered to Slateport, and he will take you to Route 109. That's your next destination.