Add a class to the first post on the first page

functions.php

add_filter( 'post_class', 'generate_first_post_class' );
function generate_first_post_class( $classes ) 
{
	global $wp_query;
	$paged = ( get_query_var( 'paged' ) ) ? get_query_var( 'paged' ) : 1;
    
	if( 0 == $wp_query->current_post && 1 == $paged )
		$classes[] = 'first';
        
	return $classes;
}