First, unzip the vBulletin zip file you downloaded from the members' area [http://members.vbulletin.com]
to your hard disk and then open the 'upload/includes/' folder. In this you will find config.php.new. You should rename this to config.php and then open it in a text editor.
The config.php file is commented throughout, so you should be able to work out what values to enter for the variables yourself. When you have finished, save the file and then upload the entire contents of the 'upload/' folder to your web server.
When this is done, point your browser at http://www.somesite.com/forums/install/install.php
is the URL of your vBulletin) and proceed to click the Next Step buttons until the script asks to fill in some addresses and names for your board.
After you have done this, the installer will ask you for some details to set you up as the administrator of the new vBulletin. A few more clicks and the script will be finished.
Before proceeding to the Admin Control Panel, you must delete the 'install/install.php' and 'install/upgrade1.php' files from your webserver. You may then enter the control panel and start working on your new vBulletin!
The entire installation process should take no more than five minutes.
For a complete description of how to install vBulletin 3, see the installation section [http://www.vbulletin.com/docs/html/install]
of the vBulletin 3 Manual [http://www.vbulletin.com/docs/html]
The values for the config.php file are described in detail here [http://www.vbulletin.com/docs/html/editconfig]