I don’t even know how to use the PSD files to create an HTML template. To me a PSD file is useless. Let’s say I want to create my own theme with this, how do I go about this ?

Themeforest is a marketplace, which consists of several sections. One of them if PSD Templates section. If you have enough coding skills and would like to build a website based on nice design, then you can purchase PSD sources and build HTML or Wordpress theme based on this design. If you do not have coding skills it is better to purchase ready HTML or Wordpress theme in appropriate section of Themeforest marketplace. 

Or, you can request a development of a theme based on the PSD or HTML template, and provide all the details in a private ticket on our support forum: http://support.bestwebsoft.com/