Practice, practice, practice. Take a practice test every couple of weeks, if you can--there are enough old tests and mock tests available in books and online for that to be pretty easy to do. eNotes even has a resource with some practice materials, which I've linked to below. You might also see if you can get a teacher, a parent, or even a friend's older sibling who did well on the test to help explain any questions you missed on old practice tests. While I don't have enough time to tutor individual students through the PSAT or SAT, as an English teacher, I've had students come to me to ask for help understanding why they missed a reading comprehension or vocabulary-related question, and I'm always happy to help with that.
Remember the purpose of the SAT, according to the College Board (the test-maker).
The SAT and SAT Subject Tests are a suite of tools designed to assess your academic readiness for college. (http://sat.collegeboard.org/about-tests)
The best way to study for the SAT is to become familiar with the test. I have included a link to the College Board’s web site. They actually have a study plan for the SAT. Begin early, and study often. Study daily or on weekends. Create a study group, and study with friends. Do as many practice tests as possible to get used to the format. The best study books are the ones that give you sample tests and explanations of the answers. College Board materials are good because you get them direct from the source.
At the second link, you can take a full SAT for free. Be sure you have a large block of time set aside, and try to take the test as if you were taking the actual SAT. This will allow you to see which areas you need to work on.
You can also sign up for the Question of the Day, and there is an app for that!
Take some practice tests out of books like Barrons or the Princeton Review. When I was studying for the SAT, I used the Princeton Review almost exclusively and found their strategies and tips very helpful. Also, ETS puts out a book with previous years' SAT test. That's a really good resource too.
You might start by checking with your school to see if they offer any SAT prep courses. Your counselor should have that information. If they don't have courses, there may be some you can find in your community, and there are definitely plenty of courses online or that you can order and study from a book.
There are sample SAT prep lesson here on enotes. I've posted one of the links below, but you can find many others as well
Beyond taking practice courses, my number one piece of advice would be to read. The more you read, the better prepared you will be. It will increase your vocabulary as well as your literary skills. Try reading things that are a bit challenging for you.
Thank you all for helping! :)