I have a html site, its hard to update since there is no backend side. so my question is hot to turn a simple html to a real website. thanks in advance
With a simple HTML template, you need to edit source files every time you wish to change content. To make things easier and dynamic, you need to use a script that stores and get data from a database. You can do this with a Content Management System (CMS) and adapt theme for it's structure. There are some good Open Source CMS that you can use , like WordPress, Joomla , Drupal and more. This are only the most popular.
I don't think that there are auto systems to convert HTML templates to a CMS theme. (but maybe I'm wrong, who knows...)
If you want to update your content using an admin panel, you're better off stripping down Wordpress and using their admin panel and some basic in-built Wordpress functions to use what you need, and display what you need to display on the front page.
The back end normally means the admin area of the site where you can change various settings.There are some simple CMS such as Pulse using which you can convert simple html pages to real websites.