add sidebar to single post

I’m trying to add a sidebar to my single posts, but I have little knowlege of what I’m doing and it’s being added to the bottom (and it looks a bit oversized).
A single post link is:
https://www.calhouncycleshopcast.com/?p=263

The singlepost code is this:

<?php include "header.php"; ?>
 <div id="container">
  <div id="topcontent"></div>
  <div id="singlecontent">
   <?php if (have_posts()) : while (have_posts()) : the_post(); ?>
    <div class="postnavigation">
     <div class="right">
      <?php next_post(' % ?','','yes') ?>
     </div>
     <div class="left">
      <?php previous_post('? %','','yes') ?>
     </div>
    </div>
    <div class="singlepost">
     <div class="title" id="post-<?php the_ID(); ?>">
      <a href="<?php the_permalink() ?>" rel="bookmark">
       <?php the_title(); ?>
      </a>
     </div>
     <h3><span class="posted"><?php _e("Posted on "); ?></span>
      <?php the_time('l j F Y',display); ?>
     </h3>
     <div class="storycontent">
      <?php the_content(__('(more...)')); ?>
     </div>
    </div>
    <?php comments_template(); ?>
    <?php endwhile; else: ?>
    <p><?php _e('Sorry, no posts matched your criteria.'); ?></p>
   <?php endif; ?>
   <div id="bottomcontent"></div>
  </div>
 </div>
<?php include('footer.php'); ?>

Thanks for any help,
Luke