To install vBulletin go to: YourURLHere/install/install.php. From there, you should get the page where it asks for your customer number, this is if you have a valid copy of vBulletin.
Edit:
In order to successfully install vBulletin, you should upgrade your PHP Configuration to Intermediate.
In order to find the database requirements, you can make it in your cPanel via MySQL.
Word of advice, I suggest you install a different forum software considering the fact that vBulletin is a resource hog. It gets horrible when you install modifications.