Once you have simple kde4 applications compiling, you know you have your development environment set up correctly and you can start fixing bugs.
The Junior Jobs page on bugzilla is a list of bugs that other developers marked as suitable for junior developers to fix. It could make a good starting point for finding bugs to fix. Alternatively you could pick your favorite application and start fixing bugs in it.
Another list of junior jobs can be found here.
The first step is to get the source of the application you need to fix.
Rest of instructions