You can't do it with html alone, you need a database. The only way to do it without a database is to subscribe to another website that offer free forums, and you just link to it. I don't know if any websites do offer that though.
So back to the question, you need to create a web application, not a web site. You need to store the discussions in the forums in a database. You need to also enable login/logout (i.e. you must save usernames and passwords in the database too)
Don't be discourage, it's not that difficult. A good way to start is to use php Nuke which is easy to use and free. (phpnuke.org)
You can also do it from scratch, but then I'd ask you to read a book about it and follows its steps.