When is the earliest point you can learn Saria’s Song

From the Zelda wikia:

Once defeated, they drop their soul-housing lanterns, allowing Link to store the spirit of the Poe in an empty bottle and bring them to the Ghost Shop in Hyrule Castle Town where they can be sold for ten Rupees. Link is also able to drink the spirit of a Poe, although doing so usually causes negative effects.

This is slightly incorrect — I spent about an hour killing/drinking Poes and only observed 3 effects (in order of frequency): +1 heart, -1 heart, and full health restoration. On the whole, soul-sucking is usually beneficial.

As well, there are 10 Big Poes throughout the game:

The hunting and collecting of Big Poes is part of a side quest. ... Once a Big Poe appears, Link must hastily dispose of it with the Fairy Bow. Once one is defeated, its spirit can be stored inside one of Link's bottles and sold to the Ghost Keeper. The Ghost Keeper will give fifty rupees and one hundred points on Link's "card" for each Big Poe collected. Once Link has gathered all ten Big Poes in the game, his card will be maxed out with 1000 points. The Ghost Keeper tells him that now he has achieved true happiness, and gives him an empty bottle for his trouble.

Here's a guide to their location, should you need it.

Once you beat Gerudo Fortress and gain the membership card (see this walkthrough for example), speak with the guard that is up the top of a ladder next to the big gate. She'll let you through to the next area.

You have to use the longshot to cross to the other side of the quicksand, and then follow the red flags until you reach a small tower.

Put on the lens of truth and follow the ghost that you can see. At the end of this area you'll arrive in the Desert Colossus.