make first home-page post 2 columns span

hi, I want to make my home page show the first post over 2 columns span, while the rest (including next pages) shows on 2 columns span.
I don’t know php, but my html and css skilles are pretty good… can someone help please?

here’s my index.php code:

<?php
/**
 * Index.php is the default template. This file is used when a more specific template can not be found to display your posts.
 *
 * @package WordPress Theme
 * @since 1.0
 */

get_header(); ?>

<div id="post" class="col span_8 clr">
    <?php
    if ( have_posts( )) :
	$att_count=0;
        while ( have_posts() ) : the_post();
			$att_count++;
			get_template_part( 'content', get_post_format() );
			if ( $att_count == 2 ) { $att_count = 0; }

        endwhile;
    endif;

    echo '<div class="clear"></div>';
    att_pagination(); ?>
</div><!-- .span_8 -->

<?php
get_sidebar();
get_footer();
?>