Starting to learn to use Hibernate is simple. The quick-start documentation on http://www.hibernate.com is easy to follow.
In the Eclipse IDE with the MyEclipse extensions, there’s also a nice configuration wizard that helps making mappings from an existing database, but sometimes that wizards does too much, so I’m back at programming the classes and mappings by hand (still quite simple while you have a nice example). The MyEclipse extensions also have a quickstart tutorial.

Basic Hibernate with one-on-one mappings from tables to objects is learned in a day. Now the object composition etc come into view. That doesn’t seem too difficult, but it’s slightly more complicated…