0) GO TO CLASS, READ THE CHAPTERS, AND DO THE HOMEWORK!
1) Go to Teacher's office hours early in the semester and Ask this question: "I know this is a really difficult class-- what are some of the common mistakes students make and how can I avoid them?"
2) If you have problems with the homework, go to Teacher's office hours. If they have any "help sessions" or "study sessions" or any thing extra, go to them.
3) Form a study group with other kids in your class.
4) Don't do the minimum...for STEM classes do extra problems. You can buy books that just have problems for calculus or physics or chemistry whatever. Watch online videos (e.g., Khan Academy) about the topic you are studying.
5) If things still are not going well, get a tutor. Your National Honor Society will have some. Or ask a teacher for a referral.
6) Read this book: How to Become a Straight-A Student: The Unconventional Strategies Real College Students Use to Score High While Studying Less by Cal Newport. It helps you with things like time management and how to figure out what to write about for a paper, etc.
7) For tests that you didn't do well on, can you evaluate what went wrong? Did you never read that topic? Did you not do the homework for it? Do you kind of remember it but forgot what to do? Then next time change the way you study...there may be a study skill center at your guidance office.
8) How much time outside of class do you spend studying/doing homework? Is it enough?
9) If you run into any social/health/family troubles (you are sick, your parents are sick, someone died, broke up with boy/girlfriend, suddenly depressed/anxiety etcetc) then immediately go to the guidance counselor and talk to them.
10) At the beginning of the semester, read the syllabus for each class. It tells you what you will be doing and when tests/HW/papers are due. Put all of that in your calendar. The teacher may remind you of things, but it is all there for you to see so take initiative and look at it.
11) Make sure you understand how to use your online class system...Login to it, read what there is for your classes, know how to upload assignments (if that is what the teacher wants).
12) If you get an assignment...make sure to read the instructions and do all the tasks on the assignment. Look at the rubric and make sure you have covered everything.
13) If you are not sure what to do, go EARLY to the teacher's office hours...not the day before the assignment is due.
14) Take advantage of any "re-do" tests you may be able to take..your teacher wants you to learn the material. Future material depends on it so you need to have the foundation. By explaining what went wrong you really understand it. Take advantage of this.