After you win the Dynamo Badge from Wattson in the Mauville Gym, you can continue north to Route 111 on your way to Mt. Chimney.
Information about the Next Gym
It will take a while to reach the next gym, but if you want to start preparing for it now, train up your Water, Ground, and Rock pokémon, because the next gym is a Fire-type gym.
Explore the Route
Be sure to teach Rock Smash to one of your pokémon. You will need it in order to get through this route.
In Route 111, if you use the Old Rod you can catch Goldeen and Magikarp. You might also encounter Geodude when using Rock Smash on the rocks here.
Go north and use Rock Smash on the rocks that are blocking the path.
Afterward, you can have a double battle with Interviewers Gabby and Ty, who challenge you to a Double Battle using Magnemite and Whismur.
North of that, you can battle Picnicker Irene, who has Plusle and Illumise.
To the north, you find Camper Travis, who has Sandshrew.
North of him, you find Backpacker Emory, who has Taillow.
There is a desert to the north, but you won't be able to go into it because of the sandstorm.
Instead, go west and you will reach the south part of Route 112.