After defeating Roxanne, you'll need to help Mr. Briney retrieve his Wingull, Peeko, from Team Aqua/Magma on Route 116. Once rescued, Mr. Briney will ferry you to Dewford Town. This small island town is home to the second Gym Leader, Brawly, who specializes in Fighting-type Pokémon. His Pokémon are strong against Rock, Ice, and Normal types, but weak against Flying, Psychic, and Fairy (though Fairy types aren't in Gen 3). Before challenging Brawly, train your Pokémon to around Level 16-19. Catching a Ralts on Route 102 or a Wingull/Pelipper can be very helpful. Defeating Brawly earns you the Knuckle Badge, allowing you to use HM03 Surf outside of battle, and TM08 Bulk Up.
