you have to use the Manager for this. You can find the JPA entity by PKey or you can use a NamedQuery which can be defined as annotation at the class with the @Entity annotation (but you must not).
Read for example the JPA tutorial http://docs.oracle.com/javaee/6/tutorial/doc/bnbpz.html.
The getter and setter of an Entity can be used in the same way as a simple class, the persistence is done by the Manager. If you read a JPA tutorial it will be clear.
If you have specific questions regarding JPA you might ask a question in one of the related forums.