When you go to Appearance –> Menus, you can actually choose which of the categories or pages you would like to display on your menu.
However, some themes don’t have this wp nav menu enabled on their theme functions.
Here is how to enable wp nav menu function:
Open functions.php and paste the following:
add_theme_support( 'menus' );
Once you have added this code, save it and open header.php or anywhere you would like this custom menu to appear and paste this code:
<?php wp_nav_menu( array( 'sort_column' => 'menu_order', 'container_class' => 'menu-header' ) ); ?>
Now go to Appearance –> Menus to choose which of the stuff you would like to display on your nav menu.