You have two choices:
1 – Keep your current site, and just link to a wordpress install for just a blog.
2 – Use wordpress as Content Management System, and migrate all your content to it.
For Option 1:
Install wordpress to some directory where it will not interfere with your main site.
Set it up (according to the docs here), do your blog, then just put a link to that blog page on your main site.
Option 2 – Migrate everything to wordpress!!!!!
This rocks, and I did this.
Step 1 - List out the features of your html site
(about pages, contact pages, etc. Basically what your current site is).
Step 2 - If your site has some special features (such as a store) check to see if wordpress can duplicate those functions (it can do almost everything).
Step 3 – Install wordpress in a subdirectory where it won’t interfere with your old site.
Step 3 - Design your new site...all on wordpress. Your static html pages (your about, other content) become 'pages' in wordpress.
Step 4 – Find a theme that matches what you want (or edit one, if you are good with php and css).
Step 5 – When new site is ready (within the subdirectory) config it to run from root.
There you go! Best of luck to you!
Insanity brings its own rewards