i would get familiar with the template tags
http://codex.wordpress.org/Template_Tags
and look at the hierarchy, to see how the templates are called
http://codex.wordpress.org/Template_Hierarchy
I would start with either twentyten, twentyeleven as a base to have a look at the code.
or i would download starkers theme.
http://starkerstheme.com/
also have a peek at chip benetts theme, it has the best wordpress coding standards out there. (code wise)
http://wordpress.org/extend/themes/oenology
hope this helps.