Highschool advice?

I know I sound fucking stupid and there are millions of websites on which I can look up advice, but I feel super comfortable in this community. I recently started high school and Im worried and uncertain about a lot of things. I want to know if high school is as horrible as people say it is. I'm just finding myself obsessing over everything, how I look, how my grades are, how people see me. I feel like it's not very healthy and all. I am genuinely curious about how much my grades really matter and how important is my image. I also want to know about everyone's experience.