Congratulations! As everyone else said, your best move right now is to relax, since it will probably be the last time you'll be able to for the next couple of years. Grad school will be great and you'll have lots of opportunities to grow, build relationships, and have fun.
Regarding dress, it often depends on not only culture but also context. Since I teach in a professional kind of program, we expect students to dress professionally anytime they are delivering services or in the community (e.g., doing trainings, consulting, etc.). In the classroom, however, they are completely casual.