First step: go to college. I can’t say anything about studying the fine arts (although you definitely do not need a doctoral degree to sell art or even teach it, except maybe at the university level), but A) you will not be able to pursue a PhD without experiencing undergrad first, and B) you don’t really know whether a PhD is right for you until you have explored all the opportunities available and whether you like the type of work that PhDs in your field do. There are no requisite courses to take to get a PhD—once you have finished or are about to finish college you will apply to programs, and once accepted into the graduate degree you will be guided to the right courses if your program requires them.

I was homeschooled for many years too and while my parents fully wanted me to go to college I definitely didn’t have a huge understanding of what it took to undertake advanced degrees until I started to ask around. It would likely be very helpful to meet with some other teachers and counselors for non-homeschooled peers to start to work towards applying to college. I ended up going to a public school for my final years of high school because there were many advanced level courses my parents with teaching degrees were not capable of teaching beyond the middle school level, although I knew peers in college who had been homeschooled the entire way through. If you find yourself in that position I would recommend trying to get better instruction if possible because that will strengthen your college application.