Hi there!
To customize your WordPress login page, you can do one of two things: (1) install a WordPress plugin or (2) edit the background yourself using CSS styles.
To install a WordPress plugin, try using the keyword “login” and search for a plugin to install.
To edit the background yourself, I suggest checking out the WordPress Codex page for Customizing the Login Form. It explains how you can add your own logo, background and stylize the login page with CSS.
Hope that helps!
