If you don’t have a site, you can use the link at the bottom of the login screen to register a new WordPress.com account and website. (If you don’t see the links, try hiding your keyboard.)
Then you can add your email, and if the email is not associated with any account it will create a brand new one. If it is, it will ask you for the password so you can log in
You can also sign up in a web browser here: https://wordpress.com/start Then, you can log in to your new account on the login screen in the app.