You need to know: HTML, CSS, little bit of PHP & vBulletin template structure. Of course, you won't be making vBulletin template from scratch, you just create a new Default style. And you modify it by adding your navbar, your forumhome styling, your header, your footer, etc.

Template structure is very easy to learn, and you need to know a little bit of PHP just so you could know if you don't want some function how to delete it, or if you want to add something and you don't know where to add it.

Best thing for you now is to go on vBulletin.com and read the manual for template structure. Of course you need to know those other things I already mentioned.