Adding a top-level admin page and sub-menu page.

add-menu-page.php

// Create a top-level page with the slug `myplugin-parent`
add_menu_page(
	__( 'My Plugin', 'my-plugin' ), 				// page title
	__( 'MyPlugin-Parent', 'my-plugin' ), 					// menu title
	'manage_options',
	'myplugin-parent',						// page slug
	'myplugin_callback',
);

// Create a sub-page under the parent page
add_submenu_page(
	'myplugin-parent',						// matches page slug above
	__( 'My Plugin Sub-Page', 'my-plugin' ),
	__( 'SubPage', 'my-plugin' ),
	'manage_options',
	'myplugin-subpage',						// page slug
	'myplugin-subpage-callback',
);