Trouble in creating a custom template on twentyeleven theme

you need to correct the automatically added body_class .singular to get the layout;

for instance use this in functions.php of your child theme (which you should have created to work from):

add_filter('body_class', 'pagetemplate_adjust_body_class', 20, 2);
function pagetemplate_adjust_body_class($wp_classes, $extra_classes) { 

if( is_page_template('new-sidebar-page-template-file-name.php') ) :
// Filter the body classes     

      foreach($wp_classes as $key => $value) {
      if ($value == 'singular') unset($wp_classes[$key]);
      }

endif;
// Add the extra classes back untouched
return array_merge($wp_classes, (array) $extra_classes );
}

if you are working with a clone of Twenty Eleven, see near the end of functions.php and expand this line with your page tempalte:

if ( is_singular() && ! is_home() && ! is_page_template( 'showcase.php' ) && ! is_page_template( 'sidebar-page.php' ) )