You’re using TwentyTen (actually, a Child Theme, I hope), right?
Here are the template files in TwentyTen:
https://core.svn.www.ads-software.com/tags/3.1/wp-content/themes/twentyten/
And here is “single.php”
https://core.svn.www.ads-software.com/tags/3.1/wp-content/themes/twentyten/single.php
Modifying “single.php” to be a one-column layout is a two-step process:
1) Modify the markup
2) Modify the CSS
Markup: remove this line:
<?php get_sidebar(); ?>
This will remove the actual sidebar column, with all its content.
CSS: change the style of <div id="container">
for single blog posts (if you want your single column to use the entire content width).
Here is the CSS declaration for #container in TwentyTen’s style.css:
https://core.svn.www.ads-software.com/tags/3.1/wp-content/themes/twentyten/style.css
#container {
float: left;
margin: 0 -240px 0 0;
width: 100%;
}
Simply remove the -240px margin-right:
#container {
margin-right: 0;
}
That should get you most of the way there!