6
Since you are familiar with html and css, you've got half the battle behind you. I'd recommend you grab several themes that interest you and study them.
The default, one of the 7danas and the UC Berkeley are good as they have different approaches.
See what they did, what you like, whether you can replicate some of their features. Move the blocks around. Try to make them fixed and non-fixed width. See what elements are common to them all.
Have fun!