I will be representing France, and the organization UNESCO. How do I prepare for it if it's my first time?

It actually depends on the topic you were assigned and the level of competition inside the committee. France usually has a very good position regarding UNESCO projects. I myself was France in UNESCO before, and they are big project supporters. The best way in which you can prepare is doing some research about your country, UNESCO itself, the topic(s) of the committee and the relevance the topic has in your country. Make summaries about the information you have, print them, and highlight what you think is relevant. Write notes. And, if you know any good delegate, talk to him, and ask him how he prepares.