Hello.
I want to start to help with Free software development, because I get motivated to add new features and improve usability of applications.

I do not know how to contribute, how the work is done and how to send a modified source code. I have never helped with any Free software development. I am a C++ programmer, I can use Qt and OpenGL, and I do not use IDE.

I have problem to understand other people's code with hundreds of files, with unknown tools and unknown patterns.

I think that I should edit KCalc as the first application.

As my first desire, I would like to improve Dolphin with a new feature or with a plugin. I think that Linux file system is too confusing and too difficult to understand. I have the idea of new features to help the user.

Thanks.

FIrst of all, welcome to KDE development!

You may be interested in some information provided on KDE TechBase, in particular the section on contributing (http://techbase.kde.org/Contribute).
To get your feet wet, hack on a simple application, produce a patch and send it to the maintainers or to the application's mailing list (if applicable) for review.

As you gain confidence with the code, you can tackle more complex tasks.

Of course feel free to ask more questions.