Hi ..
You can go into your fantastico section located in your control panel and choose to install the phpbb forum .. since your website is already on the index page and a forum requires the same, when you install the forum you will choose a directory, you can call this forum or you can call this whatever you like .. the installation process will proceed on it's own and you should be fine .. if you call it forum, you will find it by typing your domain name and adding /forum - this will be the address. You will be choosing administrative names and having this information emailed to you. Once received log into the administrative area of the forum and read, look and set up your forum. There are some neat functions that can be added to a forum and if you are interested in any of those once you figure if you can do this, let me know, could get this done for you and not alot of cost as well and if you thought the installation was too difficult, the same offer applies. Good Luck .. just remember, don't install it over on top of your main domain. If all is successful, add the link on your home page to your forum ...
www...com/forum