Guadosalam

Upon entering Guadosalam Yuna will be whisked away by Tromell. Then Rikku will teach you how to customize weapons and armor.

next you should explore the town. The Inn is right by where Tidus starts out. The town's only save point is here. Shelinda and Maechen are here as well.

The chest outside Seymour's manor contains a Mega-Potion. On the middle level you'll find a house with no items to the right and in the middle the item shop. You'll find O'aka here.

To the far left is a house where you'll find Al Bhed Primer Vol. XIII and a chest with 3000 gil.

At the foot of the last slope up you'll find a chest with an Elixir behind a pair of Guado.

Now go down to Seymour's manor. Before going to Yuna climb the stiars and look for a chest with 2 Hi-Potions on the balcony. Now talk to everyone and then enter the purple doors.

In here talk to everyone. After Seymour speaks with Yuna go outside, rejoin your party, and head up the slope to the farplane.

Inside the entry corridor you'll find a hidden treasure chest containing 8 Lightning Marbles. Then go up the stairs into the farplane.

Inside talk to Lulu and Wakka. After speaking to Wakka the Brotherhood sword gains Strength +10%, Sensor, and Waterstrike. Then talk to Yuna.

After leaving the farplane go downstairs to the red doors. After your first conversation with Lulu talk to her again. Then go talk to Rikku on the middle level. Then try to enter the item shop. After Kimahri exits the item shop go talk to Lulu again.

After this go to the exit to the Thunder Plains on the bottom level of Guadosalam.