You'll be changing this line,
<title><?php bloginfo('name'); ?> <?php if ( is_single() ) { ?> » Blog Archive <?php } ?> <?php wp_title(); ?></title>
I realize you don't know the code, but, this is how you start to learn it. Read it like English. You can see what's happening in general.
Here's the template tag (the code) used to display your Post title (example XYZ)
<?php single_post_title(); ?>
Maybe try changing in your header.php to something like this:
<title><?php bloginfo('name'); ?> <?php if ( is_single() ) { ?> <?php single_post_title(); ?> <?php } ?> <?php wp_title(); ?></title>
Then you will need to mess with it some more after that to get just exactly what you want. Or ask again for more help.
If you want to know what any of those codes / template tags does, search for it in the Codex, for example:
http://codex.wordpress.org/Template_Tags/wp_title