What kind of book/poem are you aiming for? Dramatic? Comedy? Cynical? Factual? Emotional? What type of audience would it be aimed at? (Even if no one ever reads your work, it's important to know what level of maturity the writing will be placed at.) Do you want it to be something quick and easy or a long project?
You basically have two options that I can see. Figure out the answers to the above questions, or take inspiration from a random place and let the details work themselves out. However, if you answer the questions I can take a stab at giving you some ideas. =]