Well, I cant say i've seen a good explanation through 'modifying your first template' but I cant suggest using the vBulletin Manuel (Found
here)
Modifying templates is quite easy once you get the hang on it.
You can edit your templates by going to:
www.url.com/forumdir/admin.php
Going to the bottom of the left hand navigation, selecting 'Modify' under the templates section. And the rest there is sort of self-explanitry.
Having a good knowledge in HTML is also good. Using dreamweaver isnt exactly a popular thing for vBulletin
I'm sorry that I cant be of more help. Normally I would give you the biggest tutorial ever but I am ill. But keep posting here and we'll try and do are best to help you.