Matt Wiebe gave a talk at WordCamp Winnipeg 2013 walking through the basics of theming in WordPress:
As Matt says in the intro, his talk assumes you have never built a theme and are literally starting from scratch. You’ll be looking at a bit of PHP code and HTML/CSS, but nothing too complicated. Matt does a great job of walking through the basics of making your own WordPress theme.
Your completed theme at the end might not win any design awards – but it gives you a good overview of all the files and code that goes into a WordPress theme.