Wear your favorite outfit. I always did, and it would give me that little bit of confidence that I needed to get through my first day of school, or later, my first day on the job.
What to wear on first day of 11th grade? Female, weather in the 70s and sunny, maybe semi-formal or more relaxed?
Make a statement wear a bikini
Whatever you're comfortable wearing ... Well, as long as your parents approve it and it doesn't violate any dress code. Lol