I hope this question hasn't been asked too many times, but I'll try to be original:

Most programming languages I've learned started with reading a nice book. At school, I've learned some basics about C++, but it's all simple console stuff. And I don't feel I'm learning C++ this way. I didn't have any chance to try something big, and learn why everything doesn't work


I'm thinking about playing with existing kde applications (especially kmess), creating a simple puzzle game, or writing a small control-panel applet myself. (eg. something useful for newbies). This idea is in my mind for some time, but I wouldn't know where to start. Could you please give me some pointers where to look, and what design/code rules to follow; since I want to integrate this tool with KDE as much as I can.

I think your best bet is to start reading here:
http://developer.kde.org/
Read the tutorials play with the code

Look at small kde apps check out http://freshmeat.net
for ideas and samples

As a recommendation I'd suggest getting kdevelop
if you do not have it already as it will save you from having
to write a lot of boilerplate code.

For coding my only recommendation is: KISS
If things start getting complicated you did something wrong

The big switch you are going to have to deal with isn't so
much the new functions/objects/data types but the idea
of event oriented programming. On the console things are
a bit more simple b/c you can think through the actions of a
program in a simple linear manner (for the most part anyway).
With KDE (and every GUI I'm familiar with) You react to the
and the system in a mostly asynchronous way.

Good luck...