KDE builds from Qt. If you want to build some applications in KDE, you must learn Qt at first. If you familiar with common object oriented computer programming language, likes C++ or Java, you can learn Qt easily.
If you use Microsoft Windows as your computer operating system, you can use KDE (The KDE on Windows Project). When in Apple Mac world (KDE on Mac OS) and some Linux distributions, you can use it too. Especially Linux distributions, you can find KDE in their repositories.