html template to CMS

I think the best way to go about it is first create a static Drupal template without any connections to views, blocks, etc. Do not worry about modules yet. Just create a static Drupal template and make sure it works on all browsers and appears the way you expect.

Next, for a website of 51 html pages, my best advice is to define all the common elements like your header, footer, sidebar, navigation bar etc. Once you do that, it will become clear how you can use these elements within Drupal for blocks, views, etc. Most likely, you will be using these as block regions in your .info file that you can call with a Drupal render command or you can place these elements in your Block section. All you need to do in your template's .info file is add the regions:
regions[header]
regions[footer]
regions[navigation]

Next, take out your HTML from the comment elements can build very simple Drupal blocks. Once you transfer the HTML to your blocks, edit your templates in page.tpl.php, etc and add your render statements:

print render($page['navigation']);

I won't talk about views, which are extremely powerful, but creating blocks and their regions is a great way to start.

Bruce