Can you believe that I can't think of a single good answer to this prompt?
Everything I come up with sounds like a half-assed answer from a contestant in a beauty pageant.
"I'd provide clean drinking water for all the world's population."
"I'd improve the public education system in the US."
"I'd designate many new environmental reserves and enforce measures to save our planet."
"I'd outlaw fast food and other health risks, like transfats and corn syrup."
"I'd completely change the way the meat industry and other animal-related industries function, ensuring that animals are treated humanely and not abused just to maximize profit."
I can think of a hundred such answers, but none of them are really mine, in the sense that they are generic and the type of acts we expect people in power to promise.
No, I'd want to do something truly unique for my first act as Queen of the World. Something generations would remember me for. Something that would seriously change how we function as a society...
Something like invent a kind of chocolate that has zero calories, yet still provides the exact same taste and comfort we expect from this mother of all soothing treats. Imagine if chocolate were guilt-free for the women (and men) of the world! I venture to say that we'd have a lot less depression, and a lot more enjoyment and pleasure in life.
You see, I'd really be a selfless Queen of the World, implementing something like this when I don't even like chocolate that much!