After you go east through Route 20, you will end up in Virbank City, where the next gym badge awaits.
Talk to Mom
Your Mom will give you a call as soon as you enter Virbank City. She mentions that there is some kind of complex in Virbank City.
See Roxie and her Dad
Up ahead, in front of the pokémon center, you will see Roxie arguing with her father about his dream to be both a sea captain and a movie star. He says that he can do both, because Roxie is both a gym leader and in a band. Roxie says that he's causing problems because people are waiting for him to sail his ship. He goes off to the movie studio for auditions, and she storms off to the gym to the south.
Explore Virbank City
In the Pokémon Center, you find out that Roxie uses Poison-type pokémon.
There is a blonde person standing outside of the north gate, which leads to the movie studios. In a house to the east of this gate, you find a Poké Toy.
A tough-looking guy in front of the Pokémon Gym will give you pokéballs after telling you to catch magnemites in the complex, because they will do well against Roxie's poison-type pokémon.
A girl in the preschool in the southwest part of the city will give you a Burn Heal. The trash can to the left of this house contains an Antidote.
A guy just north of the complex will give you a great ball in exchange for a pokéball. He will do this only once per day. Go north from this guy to find a hidden path that leads to a Silk Scarf at the end.