How to do the app whitch defines folder opening?

Look into hooks. They may be called something newer now but you need to create system level hooks.

First this to do is come up with a specific question to a specific problem. "I wanna" is not a question or a description of a problem.
Second. Rethink what it is that you think you want to do. What you are describing would be seriously annoying to users. Every time you open a folder, a text file opens up? Really?

To catch file system events you can read following articles:
SetWindowsHookEx function
http://msdn.microsoft.com/en-us/library/windows/desktop/ms644990%28v=vs.85%29.aspx
ReadDirectoryChangesW function
http://msdn.microsoft.com/en-us/library/windows/desktop/aa365465%28v=vs.85%29.aspx