You would need to have an .htaccess file at the top level of your servers root directory. An .htaccess file is a hidden file that executes top level commands such as forwarding an entire domain, specific pages, etc.
Also, what you'll need to know is that .htaccess files are invisible on most machines so you'll need to create and edit that file directly on your server using something like Coda (www.panic.com), which is what we use everyday to design and build sites for clients. Make sure you're able to view "invisible files" though.
WARNING... .htaccess files are VERY powerful and can be detrimental if not utilized properly.