First, don´t panic.

Second, when you start as a researcher, most professors will accept that you are far away from perfect - however they expect your hard work, your willingness to go through an intensive thinking process and so on. Typically, you will be in a more or less intensive coaching environment, where he coaches you. Of course, the majority of work has to be done by you. 

The theoretical part will grow. At the beginning, you will have intensive research of what others are doing/have done. You will read many conference papers and on that way you will learn much about your field as well as academical argumentation. If you want to start now, go to an academic library, search for Journals and conferences on your topic and start reading.

My PhD took 4.5 years, and it was not always easy, however, it was worth doing it.