If it helps, write out a little resume of the things you've done and accomplished... Like volunteer work, research, awards, maybe include your personal statement, your transcripts/grades, your hobbies, your goals. Just things your professor can physically have something to refer to when writing your letter. Other than that, just take a deep breath and be you. And ask questions. Seem like you're interested in the conversation and engage. I'm sure you'll do great! Best of luck