How do I change the HTML surrounding a particular node? I'm new to Drupal and have been tasked with fixing some HTML on a page.

There are 3 seperate sections on this page, one of which needs a couple of new classes. I've identified the section I need to change is Pane within a Node (I think). However, I cannot find what .tpl file actually influences the output of this.

How do I find out which .tpl file houses the HTML for a given Pane?
Enable theme_debug mode, once it is done you can find which .tpl file the particular section in the firebug. For enabling theme debug mode drush vset theme_debug 1

In the firebug, you can find which .tpl file the section is using.