1. You need a web host that supports PHP and MySQL
2. You should download one of the better, free-to-use forum scripts ilke phpBB click here (there are others to choose from)
3. Install the script into a sub-folder within your web structure
4. Link to it from your NetObject navigation bars
Things to be very wary of:
You have to be very, very careful about who says what on a forum. As the site owner you can find yourself in hot water if someone says something offensive, racist atc. It is classed as published content and runs very real risks for you as the site owner/manager.
An empty or low-traffic forum is pretty pathetic looking and actually detracts from your site. This is the catch 22 of including a forum on a site - until it's there people won't join it but if only a few do it looks very sparse and adds no value to your site.
Forums take a lot of work, not just in setting them up but over the long term they are sometimes a burden to run.