After seeing all the theory behind GT3, we're almost ready to start programming. However, remember you need to know Java to follow this tutorial. If you're new to Java, you will probably find the following sites interesting:
The Java Tutorial : The official tutorial from Sun, the makers of Java. Very good if you know absolutely nothing about Java.
The Coffee Break : Website with resources for Java programmers, including tutorials and FAQs.
Also, you need to be familiar with XML. You don't have to be an XML wizard, but should at least be able to read and interpret the different elements of an XML document. If you've never worked with XML, you should probably take a look at the following sites:
W3Schools XML Tutorial : Tutorial that covers both the basics and the more advanced aspects of XML.
ZVON.org : Tons of XML resources. Includes some very good reference guides.