I want to write some simple code using DDK - but i don't know even how to compile some demo code that i download.
How to compile this thing ? Is there some simple editor / IDE that i can use ? Is there some way to use visual studio to write and compile DDK ?
Generally the WDK consists of a command-line tools. No IDE or etc. However with some efforts it's possible to setup the MS standard IDE (MSVC2005/2008/2010).
First download and install the latest WDK package (from the official MS website). Go into StartMenu -> Windows Driver Kits -> WDK xxxx.xxxx.x -> Build Environments.
There you'll find a list of build environments. Each one is just a shortcut to cmd.exe, with plenty of environmental variables set accordingly. Those are build environments for different Windows version, plus for each you have a free/checked configurations, which is equivalent to user-mode's Release/Debug builds.
In order to build the driver you should launch one of those shortcuts. Then, in the command prompt, go into the source code's directory and run build batch command. The rest is determined by the makefile residing in the selected directory.
It is possible nevertheless to use the standard IDE for driver development. That is, use vcproj (or vcxproj in MSVC2010) instead of makefile. This however requires setting many build parameters.