You can check out the designer guide on this Magento page and also check out this tutorial starting from part 3. (prt 1&2 are installation etc). For me as a backend developer these 2 were very helpful in getting some insight in how templating works.
[EDIT] To implement a static html file to a theme you will first need to define which template or PHTML files hold which parts. For this you can use the Magento Template hints or Aoe TemplateHits (I'd suggest using the last one).
This way you can find the file to put your header in, menus, product blocks etc. Then copy each block that you want to edit from the
base/default template folter to your own template and customise them.
In the skins directory of your template copy the
styles.css from the
default/blank theme as it is a nice start of point from where you can add your own CSS.
I don't know any tutorial that outlines this better but these are basically the steps you need to take.