Here we go. Follow the following simple steps.

1. Login to the  wordpress dashboard as admin.

2. Click on theme editor. In wordpress release 2.8 you can get that through

Appearance —-> Editor. 

3. There at your top right corner select the theme which you want  to edit. After selection you will see list for files shown in the right corner.

4. Click on Single Post (single.php). That will open a editor. At the starting of the file,

<div id=”content”  class=”widecolumn”  role=”main”> (probably the Ln:no 10)

Change this to <div id=”content”  class=”narrowcolumn”  role=”main”>

5. At the end of the file. Just above <?php get_footer(); ?> add a new line

<?php get_sidebar(); ?>.

6. Save the file. Thats it. You will get sidebar with the selected widgets in all the posts page.

Regards,
R.Gopinath.