Decks can be earned or purchased. Using the coins, you collect in gameplay, you can buy any deck in the Deck Shop section of the deck's screen. 

You earn one new deck for each rank you reach. You reach a new rank by the total number of points you have accumulated. 

You can also earn a deck for accomplishing specific achievements. 

Lastly, decks can be earned on special dates/events. 

Clicking on a deck in the deck's screen will tell you what to do to unlock that deck.