I think I’d agree with what’s been said here, mostly. In my opinion, upgrades on things like CPU, GPU, and RAM are more “efficiency” tweaks. In other words, they’re things you can do to handle bigger scenes, faster renders, and things like that. There are other things you can do that will improve the quality of your work that are much less expensive and can be done, even if you have a weaker system.
One of the best things you can do to start improving the quality of your work is to learn to work from references. Whether this is a modeling sheet for a character, or photos of objects you want to create, or even taking something around the house and grabbing a tape measure. These sorts of things will go a long way to improving your skill, even if you have a slow machine. Also, if you want to do any sculpting in Blender, I’d recommend getting a graphics tablet, like one of the ones from Wacom. Getting things like a mirror, a tape measure, and a Wacom tablet can be less expensive than the system upgrades and will really do more to help you improve your skill.
I know everyone mentions upgrading the GPU and RAM when it comes to getting faster renders, but don’t neglect the CPU. As new features are added into Blender Cycles, they’re often not available on GPU at first. And there are some scenes where it could be possible that the GPU can’t handle what you want to give it. But again, I wouldn’t worry about any of these three at first.
I’m working primarily off of a laptop, which limits my resources, but I’m still finding it more than capable, if somewhat slowly, of handling most of the things that I throw at it.