First day? I remember it 4 years ago...
1. Be friendly, social, and level yourself with everyone. Don't treat others as more superior or as a subordinate. These are people you'll be bumping into for the four years, or more.
2. Relax, it's the first day. NO need to impress anyone, or your professor.
3. Introduce yourself, and have a sincere desire to get to know people.