Developing online web builder website using codeigniter

I am doing my internship project in reputed company, as a college internship project they ask me to develop the online web builder website.
so as fresher and very beginning step in the web development I am confused about from where to start and how others website builder works like wix.com and voog.com etc.
I am having good knowledge of PHP, HTML5, Jquery, javascript as well as good experience of codeigniter framework between training period in the company and want to develop the project using the same languages.
please guide me to understand the logic and process of this project and any help will be appreciable...