Ok you have a machine whats the next step? You know electronics, your not a machinest, what's gCode, Excellon, Gerber .... What is the process?

First some definitions:

G-Code is an EIA standard and is the basis for writing CNC control programs. G-code is comprised of simple commands that tell the machine how to move.

Excellon is a drill machine, and sometimes will see reference to a Excellon rack, which is holders for different size bits. It's a "default name thing" like cresent wrench. They were there first so the manufacturer name gets assoxiated with the data.

Gerber is the same, popular photoplotter to make films for pcb manufacturing, the defacto file format for pcb manufacturers.

The actual process of milling a pcb is typically done by converting a physical pcb design into G-code for a cnc based machine to run. Depending on how the pcb was designed, the actual process varies. For example to go from an Eagle .brd file to G-code can be accomplished by a user language program (ULP), or by generating Gerber Cam files and converting the Gerber files to G-code. You can also export an image and convert the image to G-code, or you can make a HPGL file, you can also draw a mechanical 2 d drawing such as a DXF file and convert the dxf file to G-code using a converter like ACE converter. So as you can see it can get confusing quickly. If your using Eagle, the simplest is to use a popular ULP program named PCB-GCODE. That we will explore more to make the transition simplier....:-)