How to add an HTML comment to the head of every page? I'm looking to add a copyright message to every page of my website using Drupal 7. The easiest way to do this would be to add it into the header, since that appears everywhere. I know that changes can be made to html.tpl.php.

But I'm unfamiliar (for now) with php, and need to know if I need to add any special commands to the php or can I just inject my comment into the file?
You're better off using a more Drupal-centric solution. If you're looking to place some static copyright info, you can create a block of HTML and place it in a footer region of your site's theme (under the block administration area). 

If you're just looking to play around with the Drupal CMS &amp; PHP coding in general (off topic for Drupal Answers), as long as you make your changes outside of the &lt;?php ?&gt; tags you should be fine to make changes to your template file.