31 – Olivine City
As you enter town, you will notice the Gym is right by the entrance. This is nice as you immediately know where it is. As you approach it though, your rival will come out of it. He will say he has no interest in you and complains about how the Gym Leader is attending to a sick Pokemon. He says you should go train in the Lighthouse, it might make you better. A lot of things are pointing toward that place, so it should be a good place to visit.
Follow the path as it winds into town. Head into the building just below the Gym. The person inside will ask if you want to stand and face the sea, fishing, so say “yes” to get the Good Rod. Exit the building and follow the road some more to reach both the Poke Mart and the Pokemon Center. Need be, stop by the Center and heal up your Pokemon. Talk to the girl next to the PC on the counter to battle her and see what you can do with Ball Capsules, this is Beauty Charlotte. She will send out a Bellossom lvl. 18. It will start with Sweet Scent, which will drop your Pokemon's evasiveness. It will likely follow that with Stun Spore. The people here will talk about how there is something from across the sea visiting, talking about his rare Pokemon. Talk to who you want to, then head out of the Center.
Before you visit the Mart, take a moment to visit the building to the left of the Pokemon Center, this is the Olivine Cafe. Here, the patrons will talk about how good the food is. The one at the counter will tell you that you likely could get the HM for Strength from a Mountaineer on Mt. Mortar, as they might have one to spare.
Head to the right and visit the Poke Mart.
Poke Ball, Great Ball, Potion, Super Potion, Revive, Antidote, Parylz Heal, Awakening, Burn Heal, Ice Heal, Escape Rope, Repel, Super Repel
Heart Mail, Heal Ball, Net Ball
Head down onto the docks and into the building on them. Here, you could access the SS Aqua, if it was not undergoing maintenance. If you go through the door on the left side, you will see two people just below you. If you talk to them you will see they are a couple.
If you go up the path just to the right of the Poke Mart, you will find two houses. Inside the one on the right, you will want to talk to the girl near the door, she will offer to give you some seals, if you have a seal case. In the house on the left, you will find a man offering to trade his Voltorb for a Krabby.
If you just go to the right from the Poke Mart and up the stairs, you will find yourself at the entrance to the Olivine Lighthouse. Time to go in and ascend to the top to see what you can do to help the Pokemon (and get the Gym Leader back in the Gym so you can beat them).