When you arrive in town, a man from the Poké Mart nearby will point out the blue-roofed Poké Mart. Poké Marts are shops in this game, much like the ones you found in the Pokémon Centers in the Black/White/X/Y Pokémon games: yup, they’re not in the same building anymore.
As the game goes on, the Poké Mart stock will become better. Anyhow, you’ll be given several Potions , 10 in all. These items can be used to heal 20 HP per use, which is helpful when a Pokémon Center isn’t around.
Pokémon Centers are, as the man points out, the red-roofed buildings; by speaking to the nurse within, you can fully heal your Pokémon party for their HP, PP, and ailments, and even learn if they have Pokérus! There’s also a PC in the Center that will let you store Pokémon.
When you regain control, then, visit the Center to heal your Pokémon. If you wish, you can visit the Poké Mart to buy more Potions ($200 each), but there is no real need with the 11 you already have; save your money for something else like Poké Balls further down the road. In any case, when done here, go north to Route 103 .