You need to have an account in order to participate in the leaderboards. To view the leaderboards, click on the menu button at the upper right and then on leaderboards. You can also view your specific ranking by clicking on the star icon at the top menu.
If you are not on the leaderboard press the ‘Show More’ button, scroll down and your rank and score will appear at the bottom. Please note that it takes some time for data to propagate right after you play so your scores will not refresh immediately.