From Aspertia City, head east to Route 20. Here, you'll find new wild Pokémon such as Sewaddle, Pidove, Sunkern, and Venipede. Engage in battles with trainers and catch any Pokémon that interest you to diversify your team. You'll also find some useful items scattered around the route, so explore thoroughly. Bianca will meet you on Route 20 and give you the TM27 (Return), a powerful Normal-type move whose power increases with your Pokémon's friendship.
Continue east until you reach Virbank City. Upon entering, you'll notice a large industrial area to the south. This is the Virbank Complex. It's highly recommended to explore the Virbank Complex before challenging the Gym. Inside, you'll find more trainers to battle and wild Pokémon like Magnemite, Koffing, and Growlithe (Black 2) or Elekid (White 2). Magnemite is an excellent Electric/Steel-type Pokémon that can be very useful against the upcoming Gym Leader. Defeat all the trainers in the complex to gain valuable experience and money.