How do I decide my question?

What interests you? You're going to be dedicating a chunk of your time to researching and working on this project so it is best if what you are researching interests you. It doesn't matter so much if you feel like your question is too broad or too complex, as the project goes on, you will slowly begin to refine your question naturally. Just focus on what interests you and work from there.

Perhaps, you can also choose something which relates to your post-18 choice so that you have something to talk about in your interview and you have a foundation for the start of your post-18 career/study.