A little more information might help here. What exactly do you want to do? Is your forum a part of your website, or is it your website entirely?
If it's the latter, how is it set up? Do you have the forum files in your public_html directory root, or are they in a sub directory?
To do what I think you are wanting to do, the forum files should be in a sub directory on the domain.
An example. One of our sites, the DOB Members Club, is designed solely as a forum, but we wanted to be able to add a customized front page to it. So, we put the forum into a sub directory called 'access'.
By doing things this way, we can add a customized front page to
http://dobmembersclub.com and visitors can then choose to visit the forum at
http://dobmembersclub.com/access
Bottom line, the html page you want your visitors to first see can't be in the same directory as the forum files, unless you're prepared to do some heavy editing to the php files.