In Rustboro City, you find the first Gym, where the Gym Leader Roxanne uses Rock-type pokémon. Battle her to get your first Gym Badge.
Prepare your Pokémon
Roxanne and the other trainers in her Gym use Geodude, which is Rock/Ground. Grass-type and Water-type moves are very strong against Rock/Ground pokémon. In addition, Fighting, Ground, Steel, and Ice moves are strong against Rock/Ground, but you might not yet have any pokémon that know those types of moves.
Roxanne also has a Nosepass, which is a Rock-type pokémon. It is weak to Water, Grass, Fighting, Ground, and Steel moves.
Shroomish works well in this Gym. Also, if you chose Treecko or Mudkip, it will work well here. Torchic is not a good choice for this Gym because it is weak to Rock and Ground moves. Don't use Flying pokémon or Bug pokémon (such as Taillow or Wurmple), because they are also weak to Rock moves. Seedot is strong against Rock and Ground, but it doesn't learn any Grass-type moves until it has reached a high level.
Before you reach Roxanne, you can battle trainers in the Gym, all of whom have Geodude.
In Ruby and Sapphire, there is Youngster Josh, who has a level 10 Geodude, a level 8 Geodude, and a level 6 Geodude. Then there is Youngster Tommy, who has a level 11 Geodude.
In Emerald, there is Youngster Josh, who has a level 10 Geodude. Then there is Youngster Tommy, who has two level 8 Geodudes. There is also Hiker Marc, who also has two level 8 Geodudes. Depending on where you are standing when you encounter Tommy or Marc, they might join together to have a Double Battle against you.
Roxanne is the leader of the Gym. In Ruby and Sapphire, she has a level 14 Geodude and a level 15 Nosepass. In Emerald, she has two Level 12 Geodudes and a level 15 Nosepass.
Roxanne gives you the Stone Badge after you defeat her. The Stone Badge lets you use the move Cut outside of battle. You can use this move to cut trees like the ones that you may have seen in Petalburg Woods and the northern part of Route 104. She gives you TM39 Rock Tomb, a Rock-type move that lowers the opponent's speed.
After the battle, leave the Gym and explore Rustboro City some more.