The default WordPress Twenty Eleven Theme that I use on this site by default does not have a sidebar on single posts.
The single posts will go from the example of the left to the right example
There is a plugin that will do this all for you. Use Twenty Eleven Theme Extensions instead
You will need to edit two files for the theme. You can do this in two ways:
1. Edit the files directly in
2. Go to the wordPress admin panel under Appearance>Editor option
Firstly edit single.php
at the bottom of the file change the following
to this instead
Now if you look at a single post you will see the sidebar has appeared but not in line.
Next you will need to edit functions.php
Go to the bottom on the file and find the following line
add_filter( 'body_class', 'twentyeleven_body_classes' );
Now comment this out
/** add_filter( 'body_class', 'twentyeleven_body_classes' ); */
Now the sidebar will be correctly aligned