My best advice is trial and error. Look at other styles, learn what works and try implement similar techniques into your our styles.
Try stay up to date with the latest web design news and blogs, they're always handy for tips and inspiration.
As whitemike said, turn debug mode on. This hack will let you do it more easily:
ZH - Debug Mode - vBulletin.org Forum (and let you turn it on only in the admincp).
As you said you only know a little html and css I would suggest trying to expand your knowledge here. There are many great communities like this, dedicated to html and css which can help you out along with the boat loads of free resources available.
Keep working at it and you'll get it
Hope this helps