DO YOUR HOMEWORK, seriously. You're guarenteed to pass even if you're horrible in a class if you jsut do all of your homework.
Make sure you join clubs and find a good group that you'd like to be friends with. And being friends with upperclassmen has its uses in HS sometimes for some reason
Don't get overwhelmed, It might be scary for some at first but its really not that serious. If something embarrassing happens, in reality nobody gives ****.
Also, don't get too attached to people at first, highschool is that transitional state where people where mask' all the time. Not only that but chances are high you might not talk/see some friends you made in high school after its all over.