My first day of middle school was really good. I was just as nervous as you are, my day actually turned out fine, except for those stupid annoying kids. Other than that, i had fun. And don't worry on being a loner, I made TONS of new friends.
My first day wasn't that bad, the worst part is just learning the ropes, after a week you will know how everything works. After that smooth sailing. I'm gonna be a freshman at HS when school starts, good luck!