I'm a beginner in seam. I know little about JSF and EJB, but nothing about JPA. What should I learn before seam ?..
You didn't specified what JPA version, but you can follow the Oracle's Java EE tutorials http://docs.oracle.com/javaee/5/tutorial/doc/bnbpz.html or http://docs.oracle.com/javaee/6/tutorial/doc/bnbpz.html or Hibernate Getting Started Guide