If you want one sprite you will need:

Yoshi skill
Maybe Fruit.

Once you have got these set off to the place where boats come (not In the sea!) and WITH yoshi skillfully jump onto a boat and wait until you get to and ISLAND... If you do not get onto the island try the other boat. once you get to the island with no basket quickly eat the yellow bird for a sprite!

NOTE: If you collect a sprite with yoshi he will say "Yoshi!".