The WordPress/JetPack writing prompt for today is:
If aliens arrived tomorrow, what’s the first thing you’d ask them?

Why, oh why, would aliens come to Earth?
Any reason that you might think that aliens would come to Earth is probably a trope that you got from a science fiction story.
They’re coming to conquer us.
Only if they need slave labour. If they’re coming to conquer us because we might be a threat someday, then sitting out around Jupiter and dropping asteroids on us would be the best way to get rid of us.
They would have zero casualties, and we’d never see their faces. if they did it quickly enough, we might all be dead before we even knew they were here.
They come in peace, to enroll us in some intergalactic community.
Have you seen us lately? We’re on the verge of a very stupid era of indiscriminate violence and illiterate rage. Who’d want to let us loose in the galaxy?
They’re coming for our water!
Water is one of the most abundant things in the solar system. They wouldn’t have to come to Earth to take it. Asteroids and comets are full of it, as are the polar caps of various moons and Mars.
Same with any other raw material, even diamonds.
However, there is one material that we’ve only identified as existing on Earth, and it’s possible that it is very rare in the universe: wood.
So, if aliens do arrive on Earth, and don’t just bomb us from space, then they’re here for one of two things: slaves or wood. Either way, the best question might be “Do you use Visa or MasterCard?”
But seriously…
Ok, so that’s the silly answer.
But if a species capable of interstellar travel did appear on Earth, they’re far beyond us, technologically. We would have to assume that they came for a reason and with a plan.
Our best question might just be, “How can we help you?” Because the alternative would be getting in their way.
If the idea of unknowable intelligence visiting us is your cup of tea, you might like reading Childhood’s End by Arthur C. Clarke, Story of Your Life by Ted Chiang (filmed as “Arrival”), or Roadside Picnic by Arkady and Boris Strugatsky.