Don't slack off for sure. I think as a freshmen either you will slack off and not care, or you will be a massive try hard. BEING A TRY HARD IS SO MUCH BETTER. I slacked off and it really ruined my future classes and it put me back into freshmen classes my sophomore year, and trust me when you think being with freshmen as a sophomore can't be that bad, IT'S BAD.
Second get into a sport, or get involved. Doing a sport keeps you really focused, physically and mentally. I personally recommend cheerleading, it's super fun and it's a lot more work than it seems once you get to stunting. But in general do a sport or get involved into a leadership program.
Check your diploma options ahead of time! Choose which diploma(s) you want. If you do this early you will thank yourself later!
Stay organized and BRING FOOD! There is nothing worse than starving all day long, and most teachers at this point don't care if you eat during class.
Don't let teachers mistreat you. I can't say this enough, but there will be teachers that are bullies and that's that. However don't take it, either let them know in a mature way how they're acting or tell an adult. However if they are not being equal and giving equal opportunities to everyone etc, TELL AN ADULT IT WILL EFFECT YOUR GRADE!
Challenge yourself!!!! Don't throw yourself in the easiest classes and just give yourself the minimum. But DON'T overload yourself on work, find a balance and a schedule for studying/homework and stick with it!
Free-time or seminar classes. TAKE ADVANTAGE OF THIS IF YOU HAVE IT. I have found these classes so useful. Any of the days I wanted to slack off I had this class to save me!
Last but not least, the people in high school. High school is not nearly as bad as it sounds, however that is if you are not a follower. I have watched friends, very close friends, go down the drain with their life because of someone they started hanging out with, or because they wanted to do something so and so was doing. Seriously do your own thing, and when someone questions you remind them you're not questioning them for what they do. When someone brings up your problems remind them to not bring up your problems unless they're gonna deal with their own. Keep it real with people, don't sit there and sugar-coat something to prevent someones feeling from being hurt. People would rather have hurt feelings then open up to someone who didn't like them in the first place just to be betrayed later on.
As much as I don't wanna support violence or anything, three strike rule is it. After three, they're out. For me personally I always give the three strike rule UNLESS it comes down to my family. And you will be thinking " How could anyone there disrespect my family?" Trust me on this, they will somehow. Always confront first, give it time, if it doesn't change or the LEGIT problem still occurs(not some petty issue) go for it then.
Anyways I hope this helps