When you enter your email on the first page of the app (that asks "Enter your email to begin") and then tap "Begin", the app will automatically determine whether or not there's an account associated with the provided email.
If you're forwarded to page that asks you to "Enter your password", enter the password that you remember being associated with your account. If you can't remember your password, tap the "Forgot Your Password?" option on this page. You will then be presented with an option to initiate a password reset.
However, if you're instead forwarded to a registration page, this means we have no account on file for the email that was provided. Please check the spelling on the email that you entered and consider any other email address that you may have used.