Nervous about 11th to 12th grade?

My junior year starts on August 29th and I'm worried about it and senior year. I'm worried about the changes and the SAT's and all this other stuff. I have never gone to a school football/basketball game or any athletic event. I got a 3.57 GPA for the 4th Quarter of 10th grade. Right now I am on summer vacation pondering on school. What are some good tips and advice for junior/senior year???