First head to the Pokemon Center near the entrance to Mistralton. Heal your Pokemon and buy some items if you want, then head back outside. That's right, it's exploration time! Head east and enter the red house. Talk to the girl here to receive a HP Up . Check the bin in the south-west corner to find an Iron .
Exit then head west and follow the path to the left of the Pokemon Center leading north. You'll be stopped by a woman. Does she look familiar? It's Professor Juniper!! You'll have a lovely chat with her, she'll check your Pokedex and then give you a Master Ball .
Wut? She just gave us one of the best items in the game upon meeting us for the first time? Aww yea! You don't get a lot of Master Balls, so we recommend keeping this for a super-rare Pokemon and/or one that's incredibly hard to catch. For instance, a Legendary Pokemon or a surprise Shiny Pokemon.
Shortly after, Skyla, the Mistralton City Gym Leader will appear and talk with Professor Juniper and yourself. Juniper will then leave, Skyla will talk to you, then she'll leave herself, heading back to her Gym. Her Gym's already open, so if you want to challenge her straight away, feel free to do so!
If so, skip ahead to the Mistralon City Gym section. If not, read on! Truthfully, there's not actually a whole lot to do here. Heading east at the T-intersection which take you into Route 7 and heading west will take you onto the runway, the pride and joy of the city.
Just before the runway however is a large building known as the Mistralton Cargo Service, which doubles as a control tower. Head inside. At the moment, there isn't a whole lot to do here. Talk to the woman next to the main counter and you'll receive a Sharp Beak from her.
Move to the north-east part of the room and talk to the man behind the counter. If you can show him a Flying- or Psychic-type Pokemon, he'll reward you with a Net Ball . Later on you'll be able to come back to fly to Lentimas Town, but that's a little way off just yet. Exit and head onto the runway.
First go south down the eastern side of the runway. Move east along the parked plane's south side to find TM58 - Sky Drop ! Move back to the runway and continue south until you reach the bottom of the runway. If you're playing Black 2, there will be greenhouses here growing plants within.
If you're playing White 2, these will be simple plots of soil with their contents changing depending on the season. Move between the center-left and the bottom-left greenhouses/plots to find a Max Repel . This will be obscured by the greenhouses if you're playing Black 2. Move back onto the runway.
Move up the western side of the runway this time. You'll reach a female Backpacker and talk to her. If you do, something will happen. Continue north until you come upon two small planes on the runway. Pull out your Dowsing MCHN to find a Flying Gem hidden halfway between the two.
That's it! You have two choices. First, head north and enter the Mistralton City Gym to challenge Skyla, or head east out into Route 7 to train. It's assumed that you'll train in Route 7, but if you want to skip ahead to the Gym, skip to that section.