Some of the module positions you have to create yourself in the Joomla backend. There is a guide for this following the template.
Others would have to be adding the proper image or text to have them showing as in the demo. If you are talking about stuff like the slideshow module, ja-news module and so on, these are included with the template.
Yes, it is possible to add a comment form to selected pages. Well, usually these components ( comment components ) puts comment form on each article but I think I saw a solution on extensions.joomla.org with a mambot you could add some code in the editor to display the comment form only to specific pages.
Godd luck and welcome to the club ( should you decide to join us )