Hi Emma,
Congratulations on getting an interview with McKinsey, that's great news! Don't worry about having an unconventional background - McK truly hires people from all walks of life (in fact, my mentor at McK was also an art history major!). The fact they invited you to interview means you have passed the resume screen, and beyond this your interview performance will be much more important than your major. In that sense, you are pretty much on the same footing as everyone else.
In the case interviews, you won't be assessed on your business knowledge, just on your analytical skills, so those will be crucial to develop (I'm sure you are not lacking in analytical ability at all - but regardless, this is something all candidates need to work on!). For a total beginner, I usually recommend starting by doing some reading on case interviews to get a sense of how they work, what some standard frameworks are, etc. You could take a look at some of the guides here on PrepLounge, skim through classic books like Case Interview Secrets or Case In Point, or go to McKinsey's website to look at some of their case guides. Once you've gotten a sense of what case interviews look like, I recommend getting straight into live case prep! You might not feel like you're ready, but that's ok - by far the best way to practice for case interviews is to regularly do mock cases yourself (i.e., don't just read cases by yourself - have someone, be it a coach, case partner, or friend run an actual case interview with you), and everyone faces a steep learning curve at first and then quickly gets better.
I mentioned already that you won't be assessed on your business acumen, and while that is absolutely true, it can help to have some familiarity with how various businesses work as background knowledge and to help bring the cases to live a little. If you have had little prior exposure to the business world, I'd recommend doing a bit of light research like starting to read the Wall Street Journal, or looking at McKinsey's articles to see what they've been working on. Less important than live case prep, but could still be helpful and informative.
Feel free to reach out if you have any more questions or if you'd like to chat - I've worked with lots of beginner candidates, and would be happy to help you get started too!