Recently I have completed my 12th class and have great interest in programming. I have very good knowledge of c++ but only up to class 12th level i.e. classes, structure and little bit File handling. Now I want to know how can I become a professional programmer and develop programs up to that level which we use in windows.
Please help me as.....

Once you grasp the basics of C++, you should study commonly used algorithms and data types. Linked lists, binary trees, search & sort algorithms etc. This is useful knowledge no matter what branch of programming you will settle for.

Something that is also incredible useful is knowing assembler programming, because this will give you knowledge of what goes on beneath the C++ code, and you will then become a much better programmer.

For Windows programming, I would recommend you to start with a RAD tool (rapid application development) like Visual Studio or Embarcadero C++ Builder (former Borland). These are professional tools that allows all kind of Windows programming, and they both come in completely free editions.

Once you have done some programs in a RAD tool, you may want to get down & dirty with the details of Windows programming: the Windows API. For this, I strongly recommend the book "Programming Windows" by Charles Petzold, which is the no1 Windows bible addressing almost everything in the API.

You need to learn how to build a GUI.

Graphical user interfaces usually come in Windows and Linux flavours. There are lots of toolkits for C/C++ and C#, and it is hard to say which is the best. However they all work in roughly the same way - you create windows and then hang callback functions off them to handle mouse clicks.

Get one and launch yourself into the world of graphical programming.