One of the MAIN tips any freshman needs is: do NOT, I repeat, do NOT talk/make friends with male seniors. They only want one thing. I didn't listen to this advice, and I when I realized what he wanted and refused it, he spread a rumor about me that cost me my reputation. So PLEASE for your sake, don't mess with any male seniors.
Also, don't be one of those people who are always loud and obnoxious and yelling to their friends. Because upperclassmen find that really annoying, and once your friends get that memo, they won't want to mess with someone who's super crazy, and people think you're attention seeking.
Another. Friends aren't your friends. I started freshman year with loads of 'friends.' Everywhere I turned, I had someone to chill/hang out with. But they will throw you under the bus so fast. Now, I have one friend, and she and I have been friends since middle school, AND, she didn't even go to the same school as me. So choose your group wisely. Speaking of, the people you hang out with represent you. If you hang out with people who loooove attention, who give themselves up for anyone, who are boycrazy, people are going to think that of you, and so on.
There's so much. If you wear makeup, be cautious of it, because me looking back at my makeup freshman year, I want to go back to that self and shake her shoulders and yell, "What are you dong!!!!"
As classes go, if you don't have assigned seats, choose a seat that's not in the front nor back. Go for the middle. That way, you still pay attention. And PUT YOUR BEST EFFORT INTO FRESHMAN YEAR!!! Don't e persuaded to skip classes and whatnot, because your grades matter most freshman year. If you end the year with a bad GPA, it's SUPER tough to get it back up the next few years. So please, try your best, and don't pay attention to anyone. Don't let people influence you, speaking from experience. If people call you a goody-goody, so what? You're making yourself succeed by not listening to what people say. High school is a much bigger world than middle school, so many more people to meet, so many things to join, but don't stress yourself out freshman year. You need that year to get yourself settled and used to the vibe of high school. You have 3 other years to do stuff. And lunch, lunch is the main part of the day. You don't have teachers walk you to lunch, no single-file lines to the lunch room. There's a bell/announcement to let you know it's your lunch time. Pick your table wisely, as well.
I think that's all. So, don't stress yourself out. High school is actually really fun when you get the gist of it. Make decisions for you, and not worry about what other people are going to think, because it's your life and your choice.