That's the theme default, a bit silly I know.
To modify the theme, you'll need to ensure that the template files are writeable (change the chmod permissions to 777 if you're in doubt).
(1) Go to WordPress admin > Appearance > Editor, and select "Single Post (single.php)" from the list of templates.
(2) Scroll down and add
<?php get_sidebar(); ?> directly before
<?php get_footer(); ?>. Update to save changes.
(3) Go to the "style.css" file. Search for
#primary, and change "width" from 100% to 73.6%. Update to save changes.
(4) Optional: Step 3 makes the article text itself quite thin. To change this, search for
.singular .entry-header, .singular .entry-content, .singular footer.entry-meta, .singular #comments-title and change "width" from 68.9% to 100% (or anywhere in between, depending how wide you want the article area).