How to implement easily WordPress 1.5.1 into exising website?

I have just put online a website (XHTML 1.0, CSS 2.0, PHP) and I want to integrate a news section into it, using WordPress. I installed WordPress 1.5.1 into directory /news/ on the website, but now I feel a bit lost as how to proceed – I want to use my present site design, and just integrate somehow WordPress into it.

I am not a PHP coder, a designer only.

I use only a bit of PHP to enhance the website’s functionality – for example, the header and footer are included on every page, and the navigation on the right (secondary navigation) is included dynamically, based on the section in which you are currently – if you are in, let’s say, GALLERY section, then ‘gallery-subnav.php’ is included on the right, etc.

I also use a few variables on each page (called ‘$pageID’ and ‘$pageSubID’ to change the state of the navigation tab on the navigation bars using CSS.

How can I use WordPress without making important changes to the website’s structure and design?

Thank you in advance! :-)))