Start with one of the cheap kits on Amazon. A good chunk of the learning curve is software/design/workflows. On the machine side, learning how to properly secure your work pieces, and find the right bits, speeds, and feeds is another art. You can do all of that on a ~$300 3018 CNC kit. Your work output is limited in size, and precision, but that doesn't matter as much when you're just trying to get the hang of things.