I agree with Paul, in that you need to start with JDBC. This probably implies you also need to have a firm grasp of SQL (which I assume you have if you come from the database side of development).
Can I suggest that the second step, before you look at any specific data persistence technology like ORM, JDO,
EJB CMP, SDO, is to look at the Data Access Object design
pattern?
http://java.sun.com/blueprints/corej2eepatterns/Patterns/DataAccessObject.html http://www.codefutures.com/data-access-object/ After that,
you should be able to produce (or at least generate using a tool) Java persistence code that you understand from first principles.
Then you can decide if you want to look at more advanced/complicated solutions if that is appropriate to your application.
Hope this helps....