For the most part, this isn't a WordPress question, it's a web design question. But I'll point you in the right direction:
First of all, your top navigation should look something like this in the source:
<ul>
<?php wp_list_pages('title_li=&depth=1'); ?>
</ul>
Then, your subpage nav source should look like this:
<?php
global $wp_query;
if( empty($wp_query->post->post_parent) ) {
$parent = $wp_query->post->ID;
} else {
$parent = $wp_query->post->post_parent;
} ?>
<?php if(wp_list_pages("title_li=&child_of=$parent&echo=0" )): ?>
<div id="submenu">
<ul>
<?php wp_list_pages("title_li=&child_of=$parent&depth=2" ); ?>
</ul>
</div>
<?php endif; ?>
This gives you a top level parent menu, and when you click one of the parent pages, IF IT HAS CHILD PAGES, it will display them in the sub-menu. And IF those sub-pages have child pages of their own, they will be displayed in nested ULs.
To get them to drop down, you'll want to use the suckerfish CSS technique. Here's a tutorial:
http://www.htmldog.com/articles/suckerfish/dropdowns/
Hopefully that gets you started in the right direction. But please don't ask non-WordPress questions in the WordPress support forums.
imgoel
