i know this is a bit lengthy, but hear me out, i find these really helpful
AP classes: while AP classes/credits will help you with college credits and maybe even GPA and class rank, you need to look at your self and think “do i need the AP credit.” “will this really help me?” “what score do i think i can get?” of course, everyone taking an AP class want to make a 5 on the AP exam, but be realistic, how well do you think you can do on the AP exam. im not saying that just because it’s hard you should stop trying, what im saying is that if your math grade in the past has not been ideal, and that it is very difficult to understand math concepts in the past, AP Calc might not be the class for you to take.
extracurricular classes: yes, the high leveled academic classes are very important, but this is high school. you are in the last of your years of your teens. high school could be a time for you to really think about what you want to do, what you want to major in a university. there are many exploratory classes that are offered in high school, such as: medicine, vet, preschool education, psychology, coding, journalism, engineering etc. you may not want to go into psychology in the future, but think that it is a very interesting topic, take the class. you may want to be journalist in the future, so take a newspaper class or a yearbook class to see what it is like (very stressful, but super fun. i know cause i took it). take a music class, or an art class if that interests you. this is extremely important if you dont know what you want to do in the future. and why not, these classes are free in high school (idk about private school system) where when you get to university, you have to pay a lot for these experiment with classes. another plus is that these kind of class are usually an easy A if you are really worried about GPAs.
clubs/organizations: okay, you have a million things to do and you dont want to spare time out for “unnecessary” clubs and organizations. but pick couple out that you think is interesting and go to a few of their meeting at the beginning of the school year. you may find them super fun and want to be more involved, learn something new, meet someone new. but if you dont like it, no worries, no commitment. just get out there and step out of your comfort zone, this is a really useful skill once you get to university. while you are at it, go do some volunteering work, local museum, local festival/events need volunteers, personally i think they are very fulfilling and it make me feel really good, but it also look great on your resume when applying for university.
look your best: this sounds very superficial but it will help. looking your best will help with boosting your self confidence and will really help with that stepping out of your comfort zone and make new friends. just feel good about your self just goes a long way.
study in groups: everyone says this helps with studying, and it is absolutely true. you remember better when you are interacting with the information. so study in groups, you know that you really understands the material if you can teach it to someone else, and most of the times when you didnt understand what your teacher talked about in class and if going to tutorial didnt help, it helps to have a different person explain it to you. you can also double check to see if your information/notes are correct, dont want to be studying incorrect notes