Back in the sky, I recommend you first hunt down and open the chests activated by the three Goddess Cubes you struck in recent times. The locations are on the map, by the way, but here is a quick listing anyways.
Treasure Chest 1
The first one you struck is to the southeast of Skyloft, on Pumpkin Landing. The chest yields another Adventure Pouch Slot.
Treasure Chest 2
The second chest is to the south of Skyloft on a small island. This one contains Heart Piece 11.
Treasure Chest 3
The third chest is also on Pumpkin Landing, above the entrance. You’ll need to land on the building’s roof to reach it. It contains a legendary Gold Rupee, worth 300 Rupees. Lucky you! (If you didn’t max out your wallet.) Afterwards, head back to Skyloft.
Back home, dive off near the Statue of the Goddess and go through the doorway at the base. In this room, insert the Ruby Tablet into the altar to create a fiery red beam of light, rising from the surface below to indicate the volcanic Eldin Volcano region.
Okay, we’re still in Skyloft aren’t we? Head down to the Bazaar to buy an Iron Shield and some Potion refills if you need to. Buy anything else you’ll think you’ll need.
Once you’re done, leave Skyloft on your Loftwing and fly north, dropping down into the red beam of light.