The best you can do is to add it via am i-frame. But to back up - your
server will have to offer some kind of database such as SQL or MySQL. It
will also have to support either php or asp, or something similar.
The two forum (or is that fora?) programs I am familiar with are phpBB
and Simple Machine Forum. I'm sure there are similar programs for a
Windows server.
You install and set up your forum then you can either put an i-frame on
the page with it pointing to the forum or edit the header.php file to
make it look similar to your site.
This is not as hard as it may seem, but you will need to understand the
basics of designing a web page, how php or asp works, how to set up a
database. The latter two items can be done automatically for you if your
web host offers Fantastico or Plesk.
One other thing, once your forum gets found by search engines, you will
want to limit the rights to post to only those who have registered,
usually by submitting a valid e-mail address and then activating a link
in an e-mail that will be sent. Otherwise, you'll get 15 posts a day
about Viagra and such and five posts related to your forum. SMF offers
CAPTCHA which has worked perfectly for me in the two months since I
added it.
Mike