During the course of the game, you’ll be able to find these special items called Gratitude Crystals. Much like the Skulltula Tokens of Ocarina of Time, you collect them a while until you gain a reward; however, the rewards will come much faster here. To find them, you generally finish these side-quests for people around Skyloft, although you find 15 Gratitude Crystals that are … just there.
You’ll be able to start this upon unlocking the Eldin Volcano area. After that initial side-quest, you are generally able to go around and complete the rest, except there are certain things you’ll need for most of them.