To become a Ninja in Club Penguin you have to get a black belt in the Card-Jitsu game. To do that you have to keep playing the game and win!

To get the first few belts you have to win a certain number of games before Sensei will award you the next one:

White belt = 3 wins
Yellow belt = 5 wins

Orange belt = 7 wins
Green belt = 9 wins

Blue belt = 11 wins
Red belt = 13 wins

Purple Belt = 15 wins
Brown belt = 16 wins

Black belt = 17 wins

When you have your black belt you have to play against Sensei (click on him and then choose Sensei Mode) – it takes a few goes (keep trying, he gets weaker as you go along) but when you beat him he will give you a special face mask and you will have access to the secret Ninja Room where you can play against other Ninjas:

And you will be able to buy things from the Martial Artworks Catalogue (the Ninja catalogue).

You have to be a member to be able to buy these items:

This is a video that explains Card-Jitsu – click on it and press play to see it!

Enjoy!