getting the position of a user mouse click in C & GLUT

you need to register a mouse callback function it has the following signature:

void glutMouseFunc(void (*func)(int button, int state,
                                int x, int y));

There's a tutorial that covers some basics here

Edit: If you want the position to be normalized (0.0 - 1.0) divide by the width and height:

float x1 = x /(float) width;
float y1 = y /(float) height;