You need to know C++. Read the Qt tutorials and browse the Qt docs to get familiar with what's available with Qt. Then read the KDE tutorials and browse architecture and documentation. You can also read the KDE Book, it can not harm. But you don't have to be familiar with the whole KDE architecture to become a KDE developer. Using KDE's technologies is quite easy, so concentrate on what you really need, you can learn the other bits later on.
KDE TechBase and doc.qt.digia.com (also in your $QTDIR/doc/html) are invaluable resources, take advantage of them.
Then, browse the source, look for the examples directories, see how the other did code their applications. Reading and writing code is the best way to learn.
