Creating a selecting window

Hi all.
I’m little new with OpenGl and I’m using Visual C 6. I’d like to make a window with the mouse (clicking and moving it) in order to select the part of my ogl object (or one of the ogl objects) that is inside the window itself. After the selection I’d like to crop the selected part of the object.
I think I’ve to use GL_SELECT but I can’t understand in which way.
Any idea or piece of code please?
Thanks in advance