how to edit all aspects of default theme page?

I am in the process on converting an entire site (granted it’s a simple design) over to a wordpress CMS. I wanted to just use the default template then hand code the CSS from scratch. My question is> what files does one edit in the default wordpress theme (besides of course the CSS document) to access all aspects of the page (the header, footer, body of the page etc). I looked in the documentation and couldn’t find this information.