First post full width without Sidebar

You can also doe like below code snippet:

$flag=0;
$args = array(
'orderby' => 'title',
'order'   => 'DESC',); 
$query = new WP_Query( $args );
while ( $query->have_posts() ) {
{
  the_post();
  if($flag==0)
  {
     // you first post's title, content etc
     $flag=1;
  }
  else
  {
      //rest of your post's title, sidebar, etc
  }
}