Understanding by Design
emphasizes the importance of teaching to the big ideas on a subject. PBL teachers have to be the holders of these big ideas, and help students move from a set of disparate facts to a mental map of the subject.
You have to be more expert than your students, at least for the first few weeks. Then, you need to keep learning, as student questions demand new research. One big difference between doing projects in your class and being a project-based learning teacher is that you will be going on a journey with the students as well.
Take yourself to the public library and cruise the children's section for books about the project topic. Read the local section of the paper for references to the issues that you will be studying. Try answering the driving question yourself, making a thinking map of your ideas. Put yourself in the role of lead learner.