adminstration Menus

Hi, I am new to wordpress. I would like to add a custom admin menu on dashboard page

?php add_menu_page( $page_title, $menu_title, $capability, $menu_slug, $function, $icon_url, $position ); ?>

I tried to place the code at functions.php or plugin.php page but it doesn’t work at all. it just make the site down.

any guides for correct source file and position that I can place my new menu coding correctly ?