Well I’m not sure how much you really know, but here’s what looks like a reasonable tutorial: https://kav.in/wordpress-superfish-dropdown-menu/
However, to follow that tutorial, it looks like you will need to know some basic HTML and CSS and just a little bit of jQuery.
If you’re not comfortable with that, you can stick with the script you’re currently trying to use simply by adding this anywhere to your theme’s functions.php file:
function ddsmoothmenu_js() {
wp_register_script( 'ddsmoothmenu', get_bloginfo('stylesheet_directory').'/ddsmoothmenu.js');
wp_enqueue_script( 'ddsmoothmenu' );
}
add_action('wp_enqueue_scripts', 'ddsmoothmenu_js');
To find you’re theme’s functions.php file, look in the /wp-content/themes/ folder for your theme’s name and functions.php should be in the root of that folder.
You’ll also have to upload a copy of this JavaScript file to your website in the same folder as functions.php: https://www.dynamicdrive.com/dynamicindex1/ddsmoothmenu.js