Everyone is nervous on their first day, it's natural! Do these things to ease the nerves.
1. Dress for success. You don't need to show up in a suit, but don't wear sweats either. Wear something that looks nice and will make you feel good. Peers and professors will know you take things seriously, and you are more confident when you feel like you look good!
2. Come prepared. Don't be that person that has to ask for a pen. Show up with your class materials, paper, pens/pencils etc. on the first day. Check your class website for announcements, sometimes the teacher will ask you to print the syllabus or do an assignment before the first day!
3. Wear practical shoes. If you have to be on campus all day, you don't want to wear new shoes and walk funny because your feet hurt.
4. Contrary to popular belief backpacks are acceptable!
5. Introduce yourself. Try to step out of your comfort zone and introduce yourself to professors (they like this!) and to other students. You will be less nervous once you realize your professors are friendly and that other students are nervous just like you are!