First wordpress plugin making

Solution:

Use following code

//Main admin menus
add_action('admin_menu', 'add_my_custom_menu');

function add_my_custom_menu() {

    //add an item to the menu
    add_menu_page(
        'My Plugin',
        'My Plugin',
        10,
        plugin_dir_path(__FILE__).
        'admin/plugin-form.php',
        '',
        plugin_dir_url(__FILE__).
        'img/contact.png'
    );
    add_submenu_page(
        'my-plugin-name/admin/plugin-form.php',
        'Plugin Setting',
        'Plugin Setting',
        10,
        plugin_dir_path(__FILE__).
        'admin/plugin-form.php',
        'myplugin_options_page'
    );
    add_submenu_page(
        'my-plugin-name/admin/plugin-form.php',
        'Plugin Entries',
        'Plugin Entries',
        10,
        plugin_dir_path(__FILE__).
        'admin/entries.php',
        ''
    );
}

reason is that every first sub-menu’s destination page/function will be the same as main menu destination.