To start a new game, select 'New Game' in the lower part of the screen. You will see a set of options from which to choose whether to select an opponent from your list of friends (if your account is linked to Facebook), or a random one. You'll also see the 'Invite' button, which will allow you to invite friends through different apps and the 'Refresh' button, that will add new opponents to the list.
Once you select your opponent, press 'Play'. If this is your first game, you'll get a brief tutorial that will give you the details on how to play, and how the MULTIPLICATION BONUSES and Rounds work during the game.