How To Become A Ninja

First Go to The Dojo courtyard

Enter The Dojo Talk To Sensei

He will Teach you how to Play Card-Jitsu
Remember Water-fire
fire-snow
snow-water

It takes a certain amount wins to get belts here they are

White belt - 5 total Battles needed.

Yellow belt - 13 total Battles needed.

Orange belt - 21 total Battles needed.

Green belt - 30 total Battles needed.

Blue belt - 40 total Battles needed.

Red belt - 52 total Battles needed.

Purple belt - 64 total Battles needed.

Brown belt - 76 total Battles needed.

Black belt - 88 total Battles needed.
 
After You get a black belt challenge sensei
 
It'll take a few times to beat him.
After you have beat him you will get access to the ninja hideout
 
Here is the order you get the belts: