I'm assuming then that you didn't do the vB install yourself. You'll need FTP access or the ability to file edit in the file manager of cPanel (or whatever your host uses to manage your web space). In your original vB installation archive, you'll find in the /upload/includes folder a file called config.php. This is the file you need to edit, but unless yours is already configured for your site, don't edit this one. Instead either download the one from your sites /yoursite/includes folder, edit it as I explained previously and re-upload it to your site overwriting the existing one
-OR-
Using your cPanels file manager navigate to the /includes folder and edit your config.php from there.