What could I include in this function:

MessageBox( NULL, "Dolgcas s tabo, ko si tok zabit...grem js", "Message", MB_OK | MB_ICONINFORMATION );

what can I insert instead of frist NULL?

MessageBox( NULL (here what could I insert instead of NULL), "Dolgcas s tabo, ko si tok zabit...grem js", "Message", MB_OK | MB_ICONINFORMATION );

The first parameter is the HWND of the function calling MessageBox(). When its NULL it means either the HWND doesn't exist, such as from a console program, or you don't care. If you don't know what a HWND is, then here is a good tutorial intruduction into MS-Windows programming.

so could you quickly tell me what is HWND?

If you don't know what a HWND is, then here is a good tutorial intruduction into MS-Windows programming.

so could you quickly tell me what is HWND?

Is there someting about Ancient Dragon's statement you didn't understand?

so could you quickly tell me what is HWND?

HWND is a handle to a window that you create. See the tutorial for thorough explaination.