How should I prepare my first PhD interview when I do not know about the research topic? I'm going to have my first Skype interview with the PhD committee, and I have not yet been told about the research topic. How long should the presentation be? Given that I have not been told about the research topic, how should I incline my presentation? Should I include my CV?
If you've not been told about the topic yet (though that's quite unusual - perhaps you can clarify why) then I would suggest examining the research interests of the department you are applying for. If you know the names of the interviewers, look up their research interest and their most recent papers. Are there any research groups within the department? Check for recent publications and recent (successful) grant applications. 

In addition, think about which of these research topics you could most effectively contribute to. If you had to match your expertise and knowledge so far against any of the departmental research interests, where would you fit best and where could you contribute most effectively?

In your presentation I would suggest to demonstrate that you are aware of the research profile of the institution and how you could see yourself contribute to it. Demonstrate and describe your expertise and experience backed up with evidence, and this includes summarising or presenting your CV.