e-Business Australia

It might take some time to wrap your head around the theming system but it boils down to defining a stylesheet file for the theme and at least one PHP page template based on the master ‘look’ of your site. Add PHP code in strategic places to insert the title, posts, widgets, etc. and let WordPress do the heavy lifting. Look at other themes as examples, particularly the ones that claim to be simple and CSS-oriented, they should be easier to decipher.

Everything you need to know is here:
http://codex.wordpress.org/Theme_Development