Raise a question
Strong openings withhold something. A named character doing an unexplained thing forces a reader to keep going to find out why.
Set the voice immediately
Word choice in the first sentence tells the reader whether to expect comedy, dread or warmth. Get that wrong and the rest of the page fights against it.
Cut the throat-clearing
Most first drafts begin one paragraph too early — weather, waking up, background. Delete until something actually happens.