*Disclaimer: I'm only a junior in High School and I'm still learning a lot... I am in no way an expert on life and my freshman year was by no means fantastic, which is why I'm telling you what I WISH I would've known beginning my High School career*
1. No one actually cares if you look "perfect" every day.
Stop waking up an hour earlier just to get the perfectly luscious eyelashes you're striving for, stop spending twenty minutes every morning switching your outfit and making a mess of your room in the process. You're gonna make a mess and you're gonna hate your outfit all day anyway, because you care too much about what you think other people think. They don't care or notice if you wore a shirt the second time in the last two weeks. Sleep in for once, that's more important. Wear the sweatpants, leave the house without makeup. It's fine. (Hint: Most of the upperclassmen wear hoodies and leggings everyday anyways)
2. One B- or even C on your report card is not going to end your life.
Yes, you should be striving to do your best, as always. But colleges are not going to look at your freshman year grades, see that you received one, or even two, less-than-average (for you) grade, and turn you down. You've got 3 years to prove yourself and that's plenty of time. You're putting yourself under way too much pressure; you're going to improve in the next few years, especially when you get to choose classes more tailored to your abilities and interests. You are okay, your grades are okay, just keep working hard.
3. You really don't have it all figured out, I'm sorry
You may think you know what you wanna do when you grow up and go to college and work. It is going to change at least 15 times over the course of your high school career. (That might be an exaggeration for some, but maybe not so much for some others). You're going to take classes that change your life, you're going to try new things that peak your interest, you're going to meet teachers and people that influence you in such a way that you'll change what you want to do. It's absolutely bound to happen, so just be open to it and let it come to you. Don't push new possibilities away just because you want to look like you've had it all figured out your whole high school career, because in all truth, nobody does.
4. You're probably not going to have the same friends all 4 years, and that's okay
You're probably sitting there saying "yeah right, my friends and I are gonna be together forever I have my friend group and it's not changing" which is great, if you all stay together that's awesome! But, you will change as a person, trust me. You're going to find yourself and find new things you enjoy and find new friends within those circles, and that's healthy. You NEED to have friends that enjoy the same things as you. You are going to change and grow out of people just like you grew out of shoes, but you're sure to have a handful that stick around, so keep them close.
5. You can't (and won't) understand everything
In middle school your teachers (or at least mine did) will coach you through everything, making sure you understand every last step as you go. Most classes in High School are going to be fast-paced, lecture type classes. They're going to be hard, you're going to need to take notes that you don't understand and teachers aren't always going to stop and take 10 minutes to explain everything. You're not going to understand everything right away, you're not always going to know it all; and that is perfectly fine. You're not expected to know EVERYTHING, you're a student for a reason. In fact, chances are everyone else is just as confused as you are. You've just got to adapt to being more independent and confident when you don't understand. Ask questions, get clarification, teachers are typically more than happy to help during individual work time or times outside of class... because they know you can't know everything too!