A PhD does not prepare you for a startup. I do not want to discourage you from doing a PhD if that is what you want to do (I wish I had gotten my PhD!), but do not go into it thinking it will prepare you in any way for a startup.
Find a problem that interests you, ideally one that is hugely impactful, and work on that.
Find a problem that interests you, ideally one that is hugely impactful, and work on that.