Start by going to the Olympic Games and enter Olympic Mode. Select the Toadball event on High difficulty. Follow along through the tutorials until you get to the actual event.
Toadball is a game a bit like volleyball - with your feet - and lots of balls. The object is to keep more balls on the opponents side of the net and out of your court. You do this by kicking them over the net. Press "X" for a long kick and "B" for a short kick. Only use the long kick from the back 1/3 of the court, and use the short kick from the middle 1/3 of the court. If the ball is in the front 1/3 of the court, then bring it back farther before kicking it. Only kick when you are directly facing the net.
Keep an eye out for balls that start to flash. If they start flashing, then they are about to explode, so quickly go get it and kick it over the net. If the ball explodes on your side, you will lose quite a lot of points.