Since you only want to hook into the posts on your homepage, I think you only need something like this:
<?php
add_action( 'pre_get_posts', 'my_change_sort_order');
function my_change_sort_order( $query ){
if ( ! is_admin() && $query->is_main_query() ) :
if ( is_front_page() ) :
//Set the order ASC or DESC
$query->set( 'order', 'ASC' );
//Set the orderby
$query->set( 'orderby', 'title' );
endif;
endif;
}
?>