Nacrene City

As soon as you get to the city, Cheren directs you to the Pokemon Center. He gives you 3 Chesto Berries. And he tells you that the Gym Leader uses Normal-type. Heal up, anyway-there's plenty for you to do!
In the house closest to Route 3 has a girl in it that will trade your Cottonee for a Petilil in Black, or your Petilil for a Cottonee in White. In the next house, directly left, a guy will sell you X items-X Attack, X Defend, etc. A woman in that building asks you to pick something. If you chose Snivy you get a Miracle Seed, if you chose Tepig you get a Charcoal, and if you chose Oshawott you get a Mystic Water.
As you approach the Gym, N accosts you. He talks, then asks if you agree with him. Say what you want, but heck--he's the leader of Team Plasma! Why agree with him? Anyway, he'll battle you.
He has a Pidove Lv. 13,
a Tympole Lv. 13,
and a Timburr Lv. 13.
He's not too challenging. When you beat him, he'll walk away. Before the Gym, I would suggest you go to Pinwheel Forest. You can train and catch the Pokemon N had, plus super effective Pokemon against the Gym Leader.
GO TO PINWHEEL FOREST
If not, or if you came back, challenge the Gym. When you go in, a guy talks to you. He shows you some stuff, then tells you he's the Gym Leader's husband. Anyway, go to the Gym. The puzzle is relatively easy. When you get to Lenora, save. Make sure you have a good Fighting-type Pokemon. Then battle her!
She has a Herdier Lv. 18
and a Watchdog Lv. 20.
After you beat her, her husband comes running down. Apparently Team Plasma has invaded the museum! Go and see what's up.
Team Plasma steals a fossil head. Oh no! Go outside and Burgh, Cheren, and Bianca come along. Now go with Burgh to Pinwheel Forest.
GO TO PINWHEEL FOREST