What can I start doing now in order to get flawless grades in high school?

There's no one answer to this question because everyone has different academic strengths. There's people who don't need to study and those who do. The best advice I have is to ask questions to your teachers when you have the opportunity. When you study by yourself sometimes you can miss out on what your teacher wants you to learn (Sal Khan knows a lot but he doesn't know what your teacher wants on the test). Talking to a teacher allows them to understand where you're not doing well and for them to help you in those areas. I personally think a 10 minute conversation with a teacher is just as valuable if not more valuable than two hours studying. The simple answer is just do what you need to, spend as much or as little time working outside of school to get the grade you want. For me that looks like spending less time on calculus and more on English because that's what works for me but it's different for everyone. Focus on your own needs and don't worry that you're studying too much or too little if what you're doing works.

First, count how many days you have and how much time you can spend each day to work on yourself.

Second, talk to your seniors. Ask them for suggestions and learn from their experiences and mistakes.

Third, start studying your high school syllabus. Start with the subject you wish to pursue or find easy. If you get stuck somewhere, talk to your seniors or check for online resources(Since I don't know which curriculum you study in, I can't suggest any particular website).

Fourth, practise mediation and concentration increasing exercises. In high school and in the future you might need to study longer and subjects you might not like

Fifth, work on your weaknesses this is the best time to do so. Ensure that your weak subjects don't pull down your grades in high school

Last, Think about what pushes you. If you really need to score very high then you need to be able to come over your desires and maybe sacrifice a lot of fun things. You have to come up with something which will always tell you to work harder and not to give up

the best thing you can do to set yourself up for great grades in high school has nothing to do with websites or pre-learning material or anything like that. it's much more about learning how to study effectively, manage your time well, and stay committed to your goals. those are skills you need to practice over time—so absolutely start now. if your classes are still going on, make yourself a schedule for when/how you need to study for your tests, how you're going to organize your classwork or homework, and how you plan to divide up your day into time for the different things you want to do. if you develop that discipline now, you'll have an easier time adjusting to harder work in high school.

now, if you need help or are struggling in some academic subject, khan academy is a pretty well-known and useful resource. this one is a bit more advanced, but wolfram|alpha is a very good search engine/online calculator/general study resource for specifically math and science.