I assume you've read this brief project summary: http://www.usc.edu/CSSF/History/2004/Projects/J1124.pdf
. If you haven't, please do take a look because it may inspire some ideas. After reading it, I myself was thinking of certain questions like, "What types of biodegradable plastics are there? How can I develop each type and compare them, rather than comparing a single type of biodegradable plastic with a traditional petroleum-based one?" That question led to research, through which I learned there are, very generally, two types of "biodegradables." They are 1) bioplastics made almost completely from raw and renewable materials and 2) petroleum-based plastics with certain materials mixed in that permit the plastic to be degraded, although slower than the bioplastics, I assume. This information can lead to more questions, more research, and, in turn, more questions, hopefully among them the one on which you would like to base your final project.
I understand that you've also gone to the library, but haven't found anything quite satisfactory. Sometimes the best question or the question you end up choosing is the one that comes from much investment in thought, not necessarily from books. However, for the purpose of works cited and preliminary research, you should look up books with specific
topics, such as biodegradable plastics. Engineering by itself is a broad topic, no?
Still, other Experts may have suggestions for particular books, so stay tuned!
As for the process of finding a mentor, start as soon as possible. I'm not sure if you live near a research university or labs with resources to support a project like yours, but do go on the web and Google people whose research interests match or correspond with what you want to study for the next few months. Reaching out to people means that you're enthusiastic and serious, especially given your grade level. You must remember though that unless you have immediate connections (i.e. you/your parents already know professors or researchers), you have to be adamant, patient, and never discouraged about contacting people. Waiting for responses can take days, weeks, or even months, so be on top of the ball with these sorts of things. The prior knowledge you need to know really does depend on what you end up researching, so it doesn't hurt to read papers related to the field. This definitely takes time, but if you're interested, it's absolutely worth it!
I hope this helped and wish you the best of luck with finding a project, mentor, and the right resources! And most of all, have fun!