How do you figure out the viewing frustrum?

I am new to opengl programming and am having a difficult time trying to find the viewing frustrum (ie plotting lines trying to find the closest lines to make a plane). Is there an easy way to figure it out, or get that information?

http://www.foxfiber.net/frenzy/3dclip.htm
A good source to find out stuff like the view frustrum clipping planes and clipping objects.

Antonio www.fatech.com/tech