Theming: Is this a valid way to go about it Total newbie at Drupal. Is it valid to say that I could code a page in pure html/css and then start adding the php variables needed to eventually make the page.tpl.php?

Or should I go about it by creating the tpl.php file differently?

That feels possible but all the reading I've done hasn't explicitly said it that way.
You certainly could do it that way, however, that would require you to know all of the php necessary to make a fully functional Drupal page.tpl.  What I would recommend, and how I started themeing (outside of reading a bunch!) was taking a page.tpl.php from a theme similar to what I was looking for and modifying it until I was happy with it.  After doing this several times and experimenting a lot (and reading!) I became quite familiar with the templating system used in Drupal and the php code needed in the most common template files.