I want to take the SAT Physics subject test. What should I study?

There's no rush to take SAT physics subject test until you start applying for colleges and are ready to start submitting your SAT scores to colleges you're interested in.
AP physics isn't strictly the same as the SAT subject test for physics but they will test similar things since they're drawing from the same subject.

Depending on how serious you are about the SAT physics subject test you should buy a test preparation book specifically for the SAT physics subject test. Any name brand study guide will do (Barrons, Princeton review, etc.) as long as the book includes several practice exams. Study the book, take ALL of the practice exams you can in a realistic setting similar to how you will take the real SAT. so set a timer, work in a quiet space, no cellphone, etc.
you'll start to see that many of the questions ask similar concepts and have similar formats.
Write down all the formulas you need to solve the problems in one place and memorize them. you should know them all by heart.

Do the same thing for the AP physics test when you're preparing for that. Buy a study guide. Drill the practice exams in a realistic setting. Memorize necessary formulas. Most colleges want a score of 4 or 5 on the AP in order to exempt you from the related course in college. so if you put in the effort now you won't have to take the same material again in college.

You will need to focus on stress and time management. In order to manage stress you have to manage your time wisely. <span style="background-color: transparent;">You have to set a routine for yourself and stick to it for the most part. Once you get into the groove of a routine it will be much easier for you to manage your time and have enough time for everything you need to do (including relaxing). Make yourself to-do lists on a weekly basis, use Google calendar or a planner to keep track of events, deadlines, and due dates. In addition to setting a routine and sticking to it, plan out relaxing activities into your day. Or set aside a time, after everything is done for the day, that you can have "me" time. I have also personally found it essential to not only find time for myself but also make use of that time in a way that is best for me and my holistic wellness. I have found the HeadSpace app to be an essential tool in helping me relax and generally feel more relaxed throughout the day, Guided meditation, even if you have a busy schedule, will make you feel more at ease and relaxed throughout the day as a whole (not just when you have the time to relax and focus on that "me" time).</span>

<span style="background-color: transparent;">Set a routine.Use Google Calendar.Set aside Me TimeWrite weekly to-do lists and use a planner.Find a peaceful and restful activity that will help you feel relaxed.</span>

<span style="background-color: transparent;">Here is how to be successful in them:</span>

1. <span style="background-color: transparent;">Stay on track - do assignments early, finish things a head of time, and be aware of all of your deadlines</span>
2. <span style="background-color: transparent;">do all the the assignments and read all of the coursework required- do not cut corners</span>
3. <span style="background-color: transparent;">use Google calendar to keep track of deadlines</span>
4. <span style="background-color: transparent;">communicate with your professor early if something comes up</span>
5. <span style="background-color: transparent;">do not be afraid to ask for an extension if you need one</span>
6. <span style="background-color: transparent;">study, and study a lot! you don't have regular class sessions so you will need to put in more work at home.</span>
7. <span style="background-color: transparent;">Do your best and genuinely try hard to give it your all.</span>

The best way to score high on the test is to Google search an online program or book that will help you study. Once you find an online lesson plan for success you can use it as a road map for studying. Otherwise, find a book to help you study and spend 45 mins a night reading through it and studying. Depending on when you test is, the first thing you will want to do is organize a studying schedule for yourself. Target studying certain sections of the test and divide them up by the weeks/months you have until you have to take it.

I would say definitely wait until you have at least started the AP Physics class as the AP topics and math are much more advanced. Also, your teacher may give you some sample questions for the AP tests which will be good practice for the SAT as well.