Hi,
this particular exception seems to indicate that your class Category is not a mapped entity yet. Since you are using annotations you should have at least @Entity on your Category class. I also wouldn't call the property Cat_Id since you are actually getting a instance of class Category back. The naming might be confusing to others.
Either way, I recommend to you to read/work through the online documentation. There you will also find a lot of examples -
http://www.hibernate.org/hib_docs/annotations/reference/en/html_single/.
--Hardy