First of all create the menu in wordpress. Go to appearance then menus.
then simply paste the below line on the place where you want to show new custom menu
<?php wp_nav_menu( array( ‘container_class’ => ‘menu-header’, ‘menu_id’ => ‘MENU’ ) ); ?>
In above code MENU is the name of our custom menu that we created above and ‘menu-header’ is the class that will apply on the this custom menu.