Does your web host allow anything beyond the basic html pages? The simplest way that I can think of is to use asp & a database, or php & similar but this depends on where your site is hosted. Some webspace doesn't have such features and all your beatifully crafted .asp pages wouldn't work there.
With the latter (and probably with both) there are free programs that do all the hard work for you. They often tend to say "powered by
phpBB" at the bottom, in the case of php/mySQL.