How to brainstorm in organized way

I would say you first start by listing of the CATEGORIES  or GROUPS for the exercise - then lost out specific examples. For instance, brainstorm list of capital cities in the world. You can just name then from memory... OR show structured thinking by listing out all the continents first, then start with one continent, then last cities in that and then move on to next continent. 

Hope that helps. 

I would say that it's important to be structured, even though the question is 'creative'. There are two possible situations after the interviewer asks this question: 1. You are allowed a minute to write down some thoughts: Write down some ideas that come into mind and try to group those ideas in a logical structure. That way the process for you was still free-format brainstorming, but you present it in a structured way. 2. You have to do it straight away with no time to think for yourself: I would use a similar approach, only now the brainstorming is really out loud. Name your idea, write it down and go to the next. Try to come up with groups as you go, and if you see that all of your ideas are falling in the same group/category, take a few seconds to see whether there's another category.