Just go for it.
I'm starting college soon, and it's really made me think back on what I should have done differently in high school. If I had a time machine, I would go back and undo every mistake I made. But since I can't, hopefully, I can save you from a few tears.
Quite what doesn't make you happy. Whether that's a club, a friendship, a relationship, or even a class (that isn't required), it's not worth making you miserable.
I had to come to terms that when I did things I was uncomfortable with, I was allowing my life to be run by someone else.
Don't be afraid to take chances. I waited until the second semester of my junior year of high school to audition for a play, all because I wasn't sure how to start.
I learned I really loved acting, and regretted that I had spent almost three years thinking about trying it, instead of just going for it.
People are going to come and go, and even people who you wish you never met are going to teach you something. Even though it can be really hard, try to find a lesson in every person that impacts your life. You will end up a much stronger and more compassionate person for it.
Learn how to take criticism from people who care about you. Even if the note seems a little harsh, it was given to you because someone wants you to succeed.
Try not to take it personally, and view it as a chance to make yourself and your work better. Parents, teachers, and coaches are always people you can look to for guidance and suggestion.
Try to stay uninvolved with others' relationships. Unless you're afraid your friend is in an abusive situation, it's best to keep out of it. Your friend will most likely view your input as annoying instead of helpful, despite your intentions.
Take lots of pictures, but not just to post on Instagram. There will never be another time like this in your life, and you might not see some of your friends after high school ever again.
Document as much as you can: from a rainy football game you complained about the whole time, to prom night where you watched two of your closest friends be crowned king and queen. You might not remember exactly what you were doing, but you'll never forget the way it made you feel.