If you have already created an account before, the game will ask for your password to login after you entered your email. If you forget your password, you can recover it.
To sign up with your Facebook account, simply tap the Facebook icon and give the required permissions after you log in to Facebook. Once you create an account, a temporary user name, which includes numbers and letters, will be created automatically to save your game progress and achievements. You can change it anytime from Menu > Settings > Account.