A discussion forum is not exactly a simple task. If your webserver supports Frontpage Extensions, then it's possible that Frontpage already has some support for that type of thing.
Since you're only really familiar with JavaScript and FrontPage, then you aren't really going to be able to create your own forum software. Forum software is generally written in PHP, Perl or C, and includes database acccess, either through mySQL, a similar ODBC package, or a proprietary system.
If you have your site on a server that supports PHP, I suggest looking at phpBB2, which is the software that we are using right here on WebProWrold.
If you have your site on a server that does not support PHP (or a database program) but does support Perl CGI, then there are several packages available, like YABB. Installation and configuration aren't always easy.
In the event that these options really aren't feasible, then it is possible that you could have a forum set up at a remote server location, like the services offered at Bravenet at
http://www.bravenet.com/ or EZBoard at
http://www.ezboard.com/.