viewport coordinates

OpenGL OpenGL: Basic Coding
1

If I have something like this:
glBegin(GL_POINTS);
glVertex3f(x,y,z);
glEnd();

How can I obtain the correspondent coordinates X,Y on the screen?

2

You can use the gluunproject
http://www.opengl.org/documentation/specs/man_pages/hardcopy/GL/html/glu/unproject.html

Or make the computation by hand, you have to invert the projection and model matrix.