If you're new to all this (like wot I is) you might think LinuxCNC is all you need - after all, we just got it to write its name using the mill, right?
WRONG!
LinuxCNC takes G-Code (a very low-level sort of code describing the toolpath) and spits out motor control signals for the mill's electronics. If you learn G-Code you can do some stuff by hand-coding but if you want to draw something fancy in a 3D CAD package and then make it on the mill there's a few more steps to go through.
Work in progress
Some useful links to packages can be found here: https://wiki.shapeoko.com/index.php/CAM#3D