Once you arrive, go through the first room and downstairs. Here, in the next area, you'll find the main "town square" of sorts. The shops line the northern wall. Let's speak to them.
The weapon shop offers the Tyrfing, which is superior to all other swords at this point in the game. Consider replacing your Serpent Swords. The Diamond Bell is used for Geomancers, so no point in equipping them yet, but you can purchase them to save for later if you want. There are also Fire and Ice Staves and Iron and Holy Arrows if you want to stock up on those. Take note that Holy Arrows are effective against undead foes.
The armor shop offers Ice Armor. That is a decent buy for your Warriors, Thieves, and Red Mages. Note that its Defenses are slightly lower than the Flame Mail's by one point per stat, but you'll want Fire resistance in a future dungeon. Buy some and save them for letter. The Ice Helm and Ice Shield are superior to the other Helms and Shields at this point, so give some new stuff to the Warriors, Thieves, and Red Mages (and whoever else you want). Of course, also get Kenpo Gis for Monks and Mage Robes for White and Black Mages.
Okay, once you're done, speak with the guy standing guard of the central shrine to help him out.
Now let's head to the Subterranean Lake via the southwestern passageway. Down there, you'll have to transform into a Toad to go past to the next area. Don't worry Refia, it's only temporary! You can undo the Toad spell once you're on the other side of the passageway.