To become a ninja you must play Card-Jitsu and defeat sensei. Every time you play against somebody you earn points to receive your next belt. If you are curious about your progress you can click your cards in the bottom right to see how you are doing.
Anyways when a penguin makes an account they receive a deck of cards. But before anything you must know that Fire beats Snow, Snow beats Water, and Water beats Fire. Check it out: 
There are over 9 different belts you have to earn to get to fight sensei. You have to fight other penguins and win in order to get belts.
Once you get your black belt talk to sensei and fight him. Trust me this is not easy it might take you a while to actually defeat him and become a ninja.
Once you have defeated him you can now access the ninja hideout. There you can become a fire or water ninja, but you must be a member to become these ninjas.
Good luck Grasshopper!
