To do this play a Single Player exhibition match, play Singles, select your best character, select COM2 character and make COM2 difficulty 'Novice'. When you win the message 'A new item is avalible at the item shop' will appear on screen with the item you unlocked below. If you select a star character it will be a lot easier winning the match and you can also unlock outfits in the tournament match.
To unlock different colors of the yoshi and Metal Mario, at the screen that you choose the account you want to go on, simply press UP on the control pad, and while you're doing that, press START. You will have to go on your computer (which you surely have) and look up yoshi QR codes. Scan the code you find and you will have a new YOSHI!