Who started the creation theory?

Try God the Creator, and it's not just a theory. If you mean where did it originate in the very beginning, the above answer is absolutely correct. If you mean the modern creation movement the answer is a little more complex. Many people have continued to believe in creation, despite the seeming triumph of evolutionary science among the scientific establishment. Many of the founders of modern science believed in the creator including Isaac newton, Copernicus, Keppler and Galileo among them. These and many others like them down to this present day, saw no conflict between creation and true science, as indeed there isn't.

As far as the modern creation movement goes you would look at people such as Dr.John Whitcomb and the late Dr Henry Morris who co-authored "The Genesis Flood" in 1961 as being among the most important founders of the modern movement. Of course the evidence has always been the same and so the 'creation theory' is only putting forward the abundant evidence which already exists and interpret the data from a different perspective from those who believe in evolution.