In order to use Miitomo you must have created a My Nintendo Account which was previously talked about HERE.
My Nintendo and Miitomo seems to be paving the way for the future of Nintendo, their products and addressing consumers through rewards and digital promotions similar to Club Nintendo.
There are all kinds of rewards you can gain through My Nintendo. Games, posters and more.
More information about Miitomo and My Nintendo can be found at GoNintendo.