Hi - My recommendation is, unless you are comfortable working with PHP and CSS, switch to a theme that already has horizontal page navigation built in. Getting the CSS set up correctly is a bit tricky. It would take you less time to add your present header image to a new theme than it would to convert the present theme to horizontal nav.
If you really do not want to switch, find a theme that has the horiz nav built in and borrow from its theme code, adding to your own.
The gist of it is in header.php, create code like
<?php wp_list_pages(); ?>
The coded page about horiz nav is here
then in the CSS
define the LI tags as float: left
They then get styled with foreground, background, margin, padding, etc.