After you pass through Route 101 on your way to meet Professor Birch's child, you reach Oldale Town to the north.
The building to the left with the red roof is the Pokémon Center. If you want to heal up your pokémon, go inside the Pokémon Center and talk to the woman behind the counter. She will heal your pokémon.
Outside, talk to the person who is wearing the white apron. This person takes you to the Pokémon Mart, which has a blue roof. This person gives you a free Potion after that.
In the Pokémon Mart, you can buy a Potion, Antidote, Parlyz Heal, or Awakening.
If you try going west, the scientist won't let you pass.
Continue your Journey
After you are done exploring town, go north to Route 103.
Catch Some Pokémon
Go into the Pokémon Mart and buy some Poké Balls. There are three types of pokémon that you can catch in Route 101, so get at least three Poké Balls. Then go south to Route 101 and walk around in the grass. You can catch Zigzagoon, Wurmple, and Poochyena here. To improve the chance that the Poké Ball will capture the pokémon, try reducing the pokémon's HP as much as possible before you throw the Poké Ball.