Depends on what you care about.
Im a guy, so I can only speak for guys here:
If you want to get in to a great school, take honors classes, join student govt and work your ASS off. If you take this path, I fucking commend you, I sucked at school work.
If you care about being popular/boning girls/ partying (most people do, whether theyll admit it or not) then listen up! I wish Id had someone sit down and explain all of this to me as a freshman, so Im gonna be as comprehensive as possible.
This is more for guys who might not have been top of the ladder in middle school but want to start fresh in high school and kill it.
Im gonna be totally honest and speak from my own experience, and I won't feed you any lines about "being yourself" or anything. That sounds good on a card but it isn't what will REALLY help you, and that is what I hope to do. So here it goes -
Don't try too terribly hard to be popular, or you never will be. Tryhards don't make it out there.
Don't take getting teased or made fun of too seriously. It WILL happen, and the way you react to it is what makes you a "chill" guy or not. Smile, fire back or laugh and shake your head. Don't be afraid to be a little condescending. A fun trick I picked up from a lot of kids I played football with is to call everyone "bud" or "big dog" or stuff like that. Don't be too obvious about it, but saying it in a cordial/friendly way with a smile and a handshake can be pretty ducking effective. A lot of kids fall into this trap of thinking the "popular kids" are making fun of them and freeze up or get pouty/angry. Just remember they are scared little kids too, just trying to work the sustem, and they are mindful of all the little tips and tricks too. Youre playing a game here, whether you like it or not.
Try to be nice to weird kids, but don't be too nice. They have hard enough lives but I understand sometimes you gotta join in on fucking with someone a little bit.
Make sure youre well dressed, but don't swoop anyones style because that sucks.
When it comes to girls, don't be nice. Tease them, make them feel a little bit insecure but do so playfully, acting like a sourpuss sucks and girls won't fall for that shit. Don't be afraid to text a few girls at once. When you like a girl, act FAST. Dont pussyfoot around, just be brave and ask her to hang out. Worst thing she can say is no.
Don't brag about anything, especially how much you smoke/drink/hook up. Try to be a humble guy, but don't confuse humble with meek.
Always stand up for yourself. It's better to lose a fight bravely than back down and lose people's respect (including your own.) I know that shit can be scary as fuck, but not every confrontation will be a fight. In fact, most won't, even if it gets a little physical. Just be sure you aren't letting someone punk you, fuck with you or try to make you their bitch. You owe it to yourself to make sure that person knows you can handle your shit. Don't try and get fancy if you arent a witty dude, just don't be afraid to call people on their shit.
Whatever you are doing, saying or wearing, OWN that shit. Make it yours, love it, and make other people know you aren't insecure about what you enjoy, whether it's crocs or pokemon or baseball, whatever. Ive seen this a million times - its not about what you like, its about how secure you are about liking it. If some kid tries to give you shit for your crocs (not a good choice of footwear but this is an example from something I saw) you say "whatever bro I love my crocs these are some comfy ass shoes, youre missing out!" and keep wearing them :)
Be honest. About (almost) everything. I saw tons of guys try to lie and exaggerate to make their life sound cooler or to recover from a social faux pas, but trust me, you arent as slick as you think you are. Other people see right through that shit. If you farted, own it. If you got rejected by a girl, own it. If you puked at a party, own it. If you fucked up in a way, just remember that everyone has been there, no matter how cool they think they are, and OWN IT.
Don't act impressed by kids you might think are really popular or cool. Don't offer them paper if they're out, don't try and help them or do everything they ask you to do. Don't take their side on everything either. [That one is BIG.] Some might get butthurt but at least theyll respect that you aren't grovelling for approval. That being said, don't be a dick either. Be friendly, but treat them like anyone else. If they ask for a pencil and youre in the middle of texting, tell them to hold on a minute. Take your time.
Hope this helps at least one person have a better time in high school. This info would've helped the shit out of me. Good luck fuckers!