How to add sub menu.

Hi. Im make a theme options page and I want to add admin. menu to the page with submenu. I search around but nothings found. I cant understand how to add options page from external file. Like /includes/homepage.php or /options/settings.php etc.
In my functions.php I put this code:

add_menu_page ('Page Title', $themename, 8, 'mytheme_home', get_bloginfo('template_url'). '/images/set_icon.gif');
}
    add_submenu_page('mytheme', 'General', 'General Options', 8, 'mytheme');
    add_submenu_page('mytheme', 'Portfolio','Port Options', 8, 'mytheme_port', '/includes/portfolio.php');

With this code I has a Error message :
Warning: call_user_func_array() [function.call-user-func-array]: First argument is expected to be a valid callback, '/includes/portfolio.php' was given in /home/user/public_html/wp-includes/plugin.php on line 339
And no matter what path to file error the same.

Where I’m wrong? Thank you!