Sharp as a fuckin' cue ball, this one
You need a database to store things, then you need messages and categories for messages, maybe message tags, maybe @ mentions too. You need moderators to be able to edit, delete, etc. posts. Helps if posts can be searched and they need to be shown probably newest to oldest. Then there's whatever integrations the forum has with other parts of the site. It's probably easier to pick up some forum software that already exists and reuse that, honestly. I know this site was made at the turn of the millennium, so it probably didn't have that option.
If you still want to create your own forum, it's up to you to put all this stuff together -- it's more about understanding all the database fundamentals and how to organize the back-end and front-end of a site, than it is just googling "how to make a forum". So if you don't know what you're doing, just go for existing forum software. Probably more maintainable anyway.
I got a forum software, it tends to work pretty well.
Its pretty exclusive software, but it should get you up and running.
With that said, Im not entirely sure if I should. We usually have a couple preliminary steps we have to take before we qualify someone.
I mean, I could technically show you the inner workings of all this, but it is proprietary CYScript (notice the copyright on the bottom of the page.) It doesn't work like most forums, and you could actually be sued for using the format without paying the up-front fees.
I thought this was going to be an ad.
Instead I'm just confused.