register-menu.php

<?php 

// functions.php

//Menu Register 
function wp_theme_menu(){
	register_nav_menus(array(
		'header_menu' => 'Header Menu',
	));
}
add_action('after_setup_theme', 'wp_theme_menu' );


// Menu fallback
function wp_theme_fallback_menu() {
		echo '<ul class="nav navbar-nav">';
		if ('page' != get_option('show_on_front')) {
			echo '<li><a href="'. site_url() . '/">Home</a></li>';
		}
		wp_list_pages('title_li=');
		echo '</ul>';
	}
?>



////////////////////////////////////////////////////////////////////////




<!-------

  /* html markup*/
  
	<div class="navbar-collapse collapse ">
    	
    		<ul class="nav navbar-nav">
    			<li class="active"><a href="index.html">Home</a></li>
    			<li><a href="portfolio.html">Portfolio</a></li>
    			<li><a href="blog.html">Blog</a></li>
    			<li><a href="contact.html">Contact</a></li>
    		</ul>
		
	</div>

------>


<?php
// index.php or header.php 
	<?php wp_nav_menu(array(
		'theme_location' => 'header_menu',
		'menu_class' => 'nav navbar-nav',
		'fallback_cb' => 'wp_theme_fallback_menu'
	)); ?>

?>