Pewter City

Upon entering Pewter City, your Rival will appear, mention the Pewter City Gym, give you 5 Potions, and depart.

Head north-west to the Pokemon Center to heal your Pokemon.

To the left of the Pokemon Center counter, you can speak to a man to buy Pewter Crunchies for $500. You can get one per day, and it acts as a Full Heal.

Exit the Pokemon Center and go to the south-west corner of town near an old lady.

Go north along the left side of the house, then east behind it to find and pick up an X Defense.

Backtrack to the front of the Pokemon Center.

Head north-east to the Poke Mart to purchase items.

The Poke Mart sells: Poke Ball for $100, Potion for $200, Antidote for $200, Burn Heal for $300, Ice Heal for $100, Awakening for $100, and Paralyze Heal for $300.

North-west of the Poke Mart is the museum. Entry costs $50, but contains nothing of note.

Speak to the woman with the Slowpoke to the left of the museum. Agree to look after her Slowpoke to receive a Big Pearl as a reward.

Ensure your Pokemon are fully healed and confident before heading to the Pokemon Gym.

From the woman with the Slowpoke, go south, then east into the Gym located in the center of town.