it's difficult for most people. I say use a reference and then after a while you will start to have ideas based on these. Especially for people that haven't been involved in art for a while or never have, it is almost impossible for them to know where to start. I would start with landscapes or just some architecture that you enjoy. Then make it whimsical, that is what most beginners tend to enjoy in my classes when they to don't have any idea where to start either.