To start a new game, you must press the "New Game" button located in the upper sector of the main screen.
Then you must choose your opponent, which can be random or a Facebook friend, if your account is linked to a profile.
Keep in mind that if you want to play with a friend who is not on your opponents list, you can invite him via link. Enter here to know how.
Finally, tap Play and that's it, you can start the game!