G-CODE TUTORIAL

Is there a web site that offers info on how to write G-code for a
circle? I am a newbie. I understand standard XYZ moves. I have built
a homemade CNC router using stepper motors and a dremel tool and it
works great and accuracy is plenty good for my use<.005. I am going
to use it to cut model airplane parts. Using Kcam3(very easy for me
to learn). This circle is in a wing rib and it needs to be in the
right place. I have Autocad but I will have to get training before it
will do me any good(not user friendly to me). Any help is
appreciated. Jerry Dotson