How do you know which questions to ask?
Ask a question. If it generates an idea, it was the right question to ask. If it doesn't, it wasn't; ask another one.
Repeat in perpetuity.
Imagination and creativity are closely related. Both are inherent parts of the human condition, and both can be repressed or developed as we choose.
As a kid, you probably looked up at the clouds and saw a tiger, a dragon, a freight train... or, at the very least, you looked at a shaped and polyfilled lump of fabric and saw a best friend. Try to find that part of your brain again as you look around the world. Take that man standing at the bus stop. Where did he come from? Where is he going? Does he want to go there? Does he have to? What if he simply decided not to go? What if there was a job so important that, should the person in charge not show up, civilization itself would collapse, or time cease to function? What if he was dead, and waiting for the bus that would take him to Heaven or Hell - would he stand around to find out where he was going, or would he run before he had to face his own judgement?
Sorry... got lost in the "what ifs" at the end... but hopefully you get the idea. Look at something, or someone, or even a concept (like war or politics or even household manners) and think. Really think. Ask questions. What makes this important? Why does this exist as it does? If it were different, or didn't even exist, how would things change? What if it wasn't what it looked like - what else could it be?
If you're really struggling with the idea of creativity, try reading some books on the matter. Roger Van Oech wrote a few (such as "A Whack on the Side of the Head") about learning to think "outside the box," learning how to look at problems differently and come up with original solutions; they might help.