After you win the Jet Badge from Skyla, she can take you and Professor Juniper to Lentimas Town in her plane.
Get a TM
Before you continue on to the next town, talk to the kid in the house just north of the Mistralton Pokémon Center. He'll tell you where to find their treasure. Go to the bottom left of the runway to find TM40 Aerial Ace.
Go to Lentimas Town
Go into the building to the right of the runway. Skyla and Professor Juniper should be waiting for you by the counter. If not, be sure to beat Skyla in the Mistralton Gym and go to Celestial Tower (north of Mistralton) and talk to Professor Juniper on the first floor of the tower. Then you can go back to the building next to the runway in Mistralton, and you should find Skyla and Professor Juniper waiting at the counter.
Talk to Skyla. You'll get ready to go to Lentimas Town, but Bianca will rush in and ask to go with you so she can do some research on Reversal Mountain.
Arrival in Lentimas
Professor Juniper says that she brought you here because of rumors that Team Plasma wants to control Unova with the legendary Dragons, even though those pokémon are each with a trainer. Professor Juniper wants you to go to Opelucid City to ask Drayden, a Dragon trainer, about the rumors. Agree to help her. Bianca will go to Reversal Mountain to look for the rare Fire-type pokémon that is rumored to be there.
You can find a Fire Stone to the north. You might get a call on your Xtransceiver from Curtis, the guy who lost his Xtransceiver.
In the house to the left of the pokémon center, the bald guy on the left is a Move Tutor who wants Blue Shards.
The old lady on the bottom left of the pokémon center gives you a Spell Tag.
You can get TM57 Charge Beam from the girl in the house southwest of the pokémon center (the house is to the left of the girl with the Krokorok).
When you're ready, take the east exit to go to Reversal Mountain.