current menu item in wp_nav_menu

Hi!
I want to highlight the current menu item in my menu. I am stuck.

In header.php:

<?php
			wp_nav_menu(array(
			'theme_location' => 'main',
			 'container_id' => 'navigation'
			));
		?>

in functions.php

if(function_exists('register_nav_menus')){
		//register_nav_menus(array('main-menu' => 'Main Navigation'));
		register_nav_menus(array(
		'main' => 'Navigation',
		'footer' => 'Footer Menu'));
	}

How do I continue? ??
Tips and trix are hugely appreciated!