Show the page selected in the menu

Hi everyone,

I’m creating a theme that makes a menu out of every page that is made within the WordPress Dashboard. Now, I want my theme to show a page when this page is selected in the menu (e.g.: someone clicks “Contact” in the menu, and then the Contact page opens) and this seems downright impossible to do.

I tried to make an if / else statement but it didn’t seem to make any sense. I have very little experience with WordPress and PHP, so it might just be me. Anyway, this is the code that I have so far:

First, the menu (located in header.php):
<?php wp_nav_menu (array ('items_wrap' => '%3$s') ); ?>

This is what I have to call a page, but it only calls the most recent page I have made (located in index.php):

<?php query_posts('showposts=1&post_type=page); ?>
 <?php if (have_posts()) : ?> <?php while (have_posts()) : the_post(); ?>
	<h2>
		<?php the_title(); ?></a>
	</h2>
	<?php the_content(); ?>
	<?php endwhile; " " ?>
	<?php else: " " ?>
	<?php endif; ?>

If someone could help me out, that’d be great! ^_^