To start a new game, you must press the "Play" button, located on the main screen.
Then you must choose the game mode and, finally, your opponent, which can be random or a Facebook friend (in case your account is linked to a profile).
Keep in mind that if you want to play with a friend who is not on your opponent list, you can invite them through a link. Enter here to know how.
Finally, tap "Play" and that's it, you can start the game!