You can put a .htaccess file in your www root folder. The .htaccess can perform many useful tasks, including redirecting the browser to a different URL. Just create a new text file and save it with the file name ".htaccess" -- if you use Windows, be sure to save it WITH those quotes or otherwise Windows will append .txt to the file name, and you don't want that. It's just a normal file that you can then upload to your website. The file needs to have one line that looks like this:
Redirect permanent /index.htm http://yourdomainname.com/forum/
Make sure that the last URL is WITH the entire http://... address and the first URL is WITHOUT it, like the example here.
You might want to Google for .htaccess to find loads of useful information -- or start in our DH knowledge base.