The easiest way would be to actually make an index file or .htaccess that redirects domain.com to domain.com/forum.
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
<title>My Awesome Forum</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<meta http-equiv='refresh' content='0; url=http://domain.com/forums/'>
Then just put that as index.php in the root html folder, and off you go.
Unless you're saying you want to use domain.com as the URL for your forums. Which, depending on your forum software, may require you to move it.