When you're ready, leave the Pokémon Center, and you'll find a commotion to the west. Go west, towards the crowd, where Cheren will stop you. Team Plasma is up on the hill, speaking about liberating Pokémon. Team Plasma is much like the Team Rocket (R/B/Y/G/G/S/C/HG/SS) and Team Galactic (D/P/Pt) of the previous games - they are a bad team (opposite of what you may think at this point) that resides within the Unova region.
After the speech, N, an unknown Trainer (for now), will approach and battle you.
PKMN Trainer N
Pokemon:
Purrloin (Lv. 7) Dark
Type Weaknesses and Immunities:
Purrloin has type weaknesses to Bug and Fighting, with a Psychic immunity. Unless you've been doing some serious grinding, you won't have a move of any of these types. If your Pokémon has a type move of its type (i.e., Snivy with Vine Whip), use it, as it gets a 50% power bonus (called STAB). So, this is basically a slightly-harder version of your first battle.
After defeating N, he'll speak with you for a bit and leave. Cheren soon follows suit.