Help in class diagram building

Hi to all,
I'm new in UML but I must build a class diagram about a software already developed.
The software is a plug-in for 3D modelling program named Rhino.
This plug-in read a file containing an ontology created with protege(another software) for build 3D model.
I have good information about the structure of the ontology, less information about the code.
Where can I start for make the classes? suggestion?

Thanks in advice
.