<p>I am looking for some advice on what should I concentrate my efforts to get the needed skills to become a Java developer for Oracle applications. I'm bit confused as there are lot of technologies in the Java world. Where should I start? What to avoid? Is JDeveloper a good IDE for a beginner?</p>

You should be able to do everything related to Oracle using
JDBC
, so make sure you bone up on that API.  Other than that, it depends on the type of application.  Standalone apps may use Swing (the Java UI toolkit) or in the future JavaFX, which is supposed to make Swing obsolete and may do so in a few years.  Web/enterprisey apps will make use of Java Enterprise Edition, so take a look at the servlet API, and if the app uses Enterprise JavaBeans, look at the
Java Persistence API
, which you would probably be using instead of JDBC.
I haven't used JDeveloper, but I haven't found anything wrong with the free IDEs like
Eclipse
or
Netbeans
, and my personal favorite is JetBrains's
IntelliJ IDEA
.