How can I create a simple interface for my WP plugin?

This should get you started. Once you’re more confident in plugin design I recommend using plugin boiler-plates you can create for free here: https://wppb.me/ They’ll look pretty scary at first but as you gain confidence and knowledge, you won’t be able to live without them.

Note: this is me just typing, expect typos.

    function My_Cool_Menu() {
        $icon_url = plugin_dir_url(__FILE__) . 'images/MyMenuIcon.png';
        add_menu_page('My Cool Page Title', 'My Cool Menu Title', 'edit_posts', 
        'MyPluginSlug', 'My_Cool_Plugin_main_menu', $icon_url);
    }

    function My_Cool_Plugin_main_menu() {
        //create your html admin menus, buttons and such in this file
        include_once( 'MyCoolAdminPage.php' );
    }


add_action('admin_menu', 'My_Cool_Menu');

edit and be sure you learn all about nonces!