During gameplay, press the L Button, Z Button, A and B simultaneously.
White and Black Yoshis appear as Giant Eggs, but will hatch after completing the level.
In the Trial Mode, pick the color of Yoshi that you want and press Start and Z at the same time. Then press B to go back and go to the Practice Mode, and you will have the color Yoshi you chose.
At the end of each and every Mini-Game, get enough for a heart fruit and then eat it. After all the letters appear, put them in the right positions, and you'll get a hidden message from Yoshi.