To start a new game, you must press the "Play" button, located on the main screen.
Once you do, you must choose the desired game mode and, in case of selecting "Mountain Track" or "Toad Kingdom", you will be redirected to the "Choose Opponent" screen from which you can choose a random opponent or a Facebook friend (in case your account is linked to a profile on the social network).
Finally, press "Play" and that's it, you can start the game!
We invite you to visit the article: How to play?