Return to Alder's house in the northwest part of town and you'll be able to challenge him to a battle. His Pokemon are a little tougher than Iris', but thankfully not by much, so he's a good opponent to fight right away to give you a warmup for some of the upcoming fights.
||PKMN Trainer Alder||$12400|
His team has a preference for Bug-type Pokemon, but also has a few other things to balance his types out. Fire-, Flying-, and Rock-type attacks are all generally good against his Pokemon, though. The Volcarona is definitely the star of his show, so watch out for it!
If you're having trouble taking him out, you're definitely going to want to train up your Pokemon, because there are even stronger Trainers and such up ahead. Make sure you use healing items to restore your Pokemon's HP, too, as his attacks provide a lot of type coverage.
After beating him, a young boy with similar hair will run in. It's Benga, Alder's grandson! He'll talk for a bit and then run off. Alder tells you that you can find him in either Black City or White Forest, depending on which version you're playing.
And that's that.