I'd suggest getting started with Fusion 360. Free, lots of good support, massive community and it includes the CAM element you need to generate g code.

Loads of useful youtube content to get you started. Look up Lars Christensen, NYCCNC, Titans of CNC etc etc - plus the Fusion 360 team itself.

Watch some vids, install the software, have a play. That's how it starts - before you know it you'll be buying all sorts of stuff you don't need!