opengl camera

Hey, how would I create an opengl camera facing down, with the ability to zoom in / out?

I created a zoom with the mouse wheel, but this really only zooms a specific image, so its fake :)

I;d like to be able to zoom in on the whole 2d scene, or zoom out a bit. With the mouse wheel of course.

glFrustum -zoomX,ZoomX,-ZoomY,ZoomY,.1,10000;

thanks ill try it