After you learn to catch pokémon in Avance Trail, go north into Santalune Forest.
When you first enter the forest, Shauna will come up and decide to follow you. Talk to her any time and she will heal your pokémon for you.
In the tall grass of Santalune Forest you can find Caterpie, Metapod (Y only), Weedle, Kakuna (X only), Pikachu, Pansage, Pansear, Panpour, Fletchling, and Scatterbug.
To the right of the entrance, there is an item on the ground. Check on it. It's a potion. Go to the left and you will find a kid who mentions that this is a dead end. Go left from the kid to find an Antidote past some tall grass. Go north from the kid and a little to the left to get a Potion just past the grass.
To the east, you find your neighbor, Trevor, and Tierno looking around for pokémon in the tall grass. Just north of Tierno, walk into the grass and Shauna will find a Paralyze Heal hidden in the grass.
North of there, you find Youngster Joey, who challenges you to a battle. He has a level 3 Scatterbug and a level 3 Fletchling.
Go west from Youngster Joey, then south. Follow the path and you eventually reach your neighbor. Talk to your neighbor to receive a pokéball.
After you go past your neighbor, you encounter Lass Anna. She has a level 5 Pikachu.
South of her, you find Lass Lise, who has a level level 2 Weedle and level 4 Bunnelby. You get 96 Pokédollars for winning.
Go east from her to find a Poké Ball. Go west from her to find a Potion in the grass.
Go north from there. Your neighbor will come up to you and recognize your potential. Then the rest of your friends will arrive. Go north to Route 3 Ouvert Way.