How / where to place my own custom PHP header and navigation, above Zen Cart

I have a website that I've installed Zen Cart on, and i'm having difficulty getting my own header to sit on top of everything.

What I have is pretty simple, there is a <?php require($_SERVER['DOCUMENT_ROOT'].'/'.'init.php'); ?> that I use at the beginning of my page, and a <?php showStandardHeader(true); ?> to call up my header (this is what brings the header up on my site)

Where would I place my code to get my header to appear in Zen Cart? I'm just looking to set my own navigation on top of the Zen Cart - hoping someone can steer me in the right direction, I've been searching the forum but with no luck yet. Thanks in advance.

Have a look at includes/templates/template_default/common/tpl_header.php and make a copy for your template - you should be able to insert your code in the proper order there