Don't let the upperclassmen intimidate you
Movies compare high school to a food pyramid. The seniors are at the top as the lions while the freshmen are at the very bottom as the poor little rabbits about to get demolished. Truth is most high schools are NOT like this. Some of my best friends when I was a freshmen were seniors. I'm not saying all schools are like this but most aren't. As a senior I can say I actually don't mind most of the freshmen, I barely even notice that they're there.
You're not going to keep the friends you had in middle school
Now this is not always the case but sadly most of the time it is. Not every friendship that ends will end badly, sometimes its just impossible to maintain the friendship. Friends move to different schools, have different classes, meet other people and sometimes you and your friend just grow apart. It sucks but it's a part of high school. In my personal experience most of my friends I just grew apart from because we had different goals in life or they weren't a good influence for me.
But you will always have that one friend that sticks by your side
Although you may lose most of the friends you went to middle school with there is almost always that one friend that is always going to be there for you. Whether you talk to them everyday or once a week they will always come through if you need them. This has remained true for me and my best friend since 6th grade. We don't talk everyday but he's always there if I need him and I'm always there if he needs me.
Manage your time and actually do your homework
Teachers aren't lying when they tell you that you NEED to do their homework. Its true. Along with homework goes time management. If you create a schedule and include an hour for your homework I can guarantee you will pass all of your classes. In school I rarely missed a homework assignment and I managed to pass every single class solely because I did my homework and at least tried to do well on exams.
Respect and be respected
If you respect your teachers they will respect you. If you do what you're supposed to and don't talk back you will never have a problem with a teacher unless they're just an absolute asshole. You're going to need your teachers by the time you're a senior to write you recommendation letters for college. So just be nice to them.
Participate in at least one activity at school
This can mean join a sports team, try out for the school play or join the debate team. I strongly advise you to join at least one club, sport or organization. It helps you make friends and it makes it easier to adapt to a new setting. It will not only allow you to meet other people with similar interests to you but you can also use this when it comes time to go to college or get a job. Employers/ colleges love when students do other activities, it shows you have motivation and drive.
Don't worry about getting into a relationship
It will happen when it happens. Listen to your parents when they say not to get involved with someone because they're right. High school is not a time to focus on finding "the one". That person will come when the time is right. As a student you already have so much to focus on and its so overwhelming to try to maintain a relationship and a being a student and an athlete and whatever else you do. Most relationships don't last after high school anyway due to the long distance and barely seeing each other. Not to say that if you do get into a relationship during HS it won't last because it just might be the love of your life, but this is direct to those who think that you have to find a significant other in high school. Most people aren't even mature enough to be in a relationship at this age. Its okay to be single :)
Believe in yourself
This sounds so cliché but always believe in yourself. tell yourself that you will pass that really hard AP class, that you will get through the year, that you will make tryouts. If you fill your head with negative thoughts, you're going to have a negative outcome. So think POSITIVE!
Take that AP or honors class you want to take but are too afraid of failing. Odds are if you do your homework and manage your time you'll be perfectly fine. By the time you're applying to colleges you will have a huge advantage over all the other students who took regular classes. Yes it may be harder and more fast paced but it helps you prepare for college and it gives you course credit most times so you're saving money. (Disclaimer: it is perfectly okay to take regular classes, I took mostly regular classes and I am doing just fine. I'm just advising you to take harder classes to get you out of your comfort zone and help prepare you.)
Stay true to yourself
Don't let your peers influence what you do and say. If your friends tell you to drink or smoke and that's not something you feel comfortable with, DO NOT DO IT. It is so much better to stick to your morals and values than to listen to those around you and feel miserable afterwards. If someone tries to tell you that you wont be cool if you aren't doing what they're doing or wearing what they wear, then be uncool. Being uncool and happy with yourself is a million times better than being "cool" and being unhappy. (If any of your "friends" are like that, then they aren't your friends :/)
I hope these tips/ guide helps at least one person have a better high school experience. Don't focus on the bad things that happen because by the time you go to college it won't matter. Always be true to yourself and remember that its okay if you're not the smartest, the most athletic, have a lot of friends because as long as you're content with what you do and who you are that's all that matters. Good Luck guys!