Ok
I once made a small forum.. it was fairly easy and made it within a few days.
1st.. you will need to set up tables for the categories and sub categories of the forum. Then you will need to set up a table for the topics (which means not for the replies.. but only topics).. then you need to create a table called posts... here will include every post made on the forum.
The reason for topics as one table and posts as another.. is so that you can assign the posts to go to whichever topic... 🙂
Also.. you will need to set up user login and profile and stuff like that.
That is just a basic forum.. Notice I didn't explain how to use it all.. but maybe that will help you a bit.