After you make your way through Route 213, you reach Pastoria City, home to the fourth gym.
Get a Scarf
In the first house from the east entrance of Pastoria City, you can talk to the green-haired Ace Trainer to get a scarf for the first pokemon in your party, but only if that pokémon's condition is good. You can increase its condition with Poffins.
Go outside and talk to the person with the pink umbrella to get Flora Stickers C.
East of the Pastoria Gym, there are some berry trees.
In the house south of the berry trees, you will find a Move Tutor. You can exchange Heart Scales to teach your pokémon moves.
In the water southeast of the Move Tutor's house, you can use the Good Rod to catch Magikarp and Remoraid, or use the Old Rod to catch Magikarp.
Get a Berry
In the house west of the Poké Mart, talk to the people there to get a berry.
Get a Full Heal
Go southwest from there and check in the puddle to find a hidden Full Heal.
You can now visit the Great Marsh using the north gate in Pastoria City.
When you are ready, enter the Pastoria Gym.