A good question must have only one specific answer.
For example if you ask the question, "Who played James Bond in the movies?" there are lots of answers. But if you ask "who played James Bond in Goldfinger?" there's only one answer.

Base your question on a fact not an opinion.
If you ask "who's the greatest basketball player of all time" there's lots of opinions, but no single answer. If you ask "what NBA player scored the most points ever in one game?" that's a fact, not an opinion.

Your questions should be based on a fact that is common knowledge.
It shouldn't be a fact that's so obscure that most people would have no idea. "Who won the Oscar for Best Actor in 1938?" Too Hard. If half of the people you know would have absolutely no idea of the answer to your question, then it's probably too difficult and not fair for the show.

The fact the question is based on must be able to be found in reputable books, magazines or newspapers.
You can't ask a question like "who was my third grade teacher?" It's not fair. Only you and a few friends know the answer.