Bringing in a header with PHP

My site is live at ynutah.com. I hand coded this site, and I am needing to bring the navigation (which is its own file “navigation.php”) into the wordpress end of things. My wordpress site is the blog. The URL is blog.ynutah.com

What is the correct way to make this connection if the wordpress is installed on a sub domain? I can make it so the header appears in HTML/CSS, but then when I want to make a change I have to remember to change it in the main php navigation file, as well as the blog header file, and I don’t want to have to keep doing that. I don’t know if I’m linking it wrong, but I can’t get it to work.

Thanks in advance.