I suppose step one would be a smartly designed forum database, either in Access or SQL Server (among others).
There are some examples on the web...try googling 'Snitz Forum'. It should be freely downloadable still.
Up front, you need to consider how you will present the data, like:
Single forum or multi-category forum?
Thread posts in hierarchy or in straight-line from original post?
Administration needs to OK posts before they display?
Need to register to post? Registered posts and guest posts allowed?
Above all, if you just want to do this and don't really want to dive in head first, download a free one from the web and customize it. Or, fess up a couple hundred bucks for a ready-made app. Or, go PHP, pay for site hosting, and the host usually has ready-made forums (guestbooks) that you can use (shop around).