How can I respond to an anti-Islam website that asks for proof that the Qur'an is the Book of Allah, when I am not sufficiently knowledgeable about the subject?

Do not engage in refuting misconceptions as long as you are unqualified to do so, lest they take root in your heart. Occupy yourself with learning the correct Islamic creed from the books of the righteous predecessors, then learn what is obligatory for you concerning fiqh (Islamic jurisprudence) and transactions. After you become qualified, you can defend the religion, for a good intention alone is not enough. Leave this matter to the specialists, and trust that Allah protects His religion from the extremism of the overzealous and the distortion of the fabricators. He has appointed those who will refute their falsehood. Allah Almighty said: "They want to extinguish the light of Allah with their mouths, but Allah refuses except to perfect His light, even if the disbelievers dislike it."