Here's what I wish someone would have told me when I was in high school. First do all of the Fusion 360 tutorials on the Autodesk learning center. Really understand user parameters, add ins, and excel to create simple scripts. Next, learn everything you can about tooling, understand the Shars catalog from beginning to end, know what a difference a Mitsubishi tool makes over some cheap junk. Next learn CNC Programming, learn to effectively use patterns, Macros and create menus. Learn Jig and fixture design. This should certainly be done at a company that is doing things the toyota way. Read 4 books, "The Goal" "My Life and Work" "The Toyota Way" and "Self Help" Reread these books again down the road. Finally after all that it should be very clear for what you should do next, Start with something like a used Fanuc Robodrill or Brother Tapping machine or Haas DT1. There are toons more boring parts in the world like vise jaws, fixture brackets, urinal cake molds, and tooling plates that need to be made. If you've setup a shop like with great business principles and have bought a fast and high quality machine then you'll have no problem making tons of money.

The goal is to make money, and goofing around with a DIY CNC mill is not going to get you any closer to making money. But if someone told me that I still would have gone down the road of retrofitting an old Bridgeport Series 2 CNC to have a LinuxCNC control, then bought a Fadal.