Although most people take the SAT in their junior year of high school, the best time to start preparing for is in your Sophomore year, and the summer before your Junior Year. In my experience, students take on a lot more responsibilites in their Junior Year (learning to drive, more AP classes, researching colleges, etc.), so you don't want to wait until you are the midst of your Junior Year to start studying.
Many cities, including Seattle, have numerous choices for SAT Prep Courses you can take, and there are tons of books out there,but if you are looking for a free way to start your studies, I highly recommend using the SAT prep resources on Khan Academy.
On Khan Academy, you can link your College Board account to your Khan Academy account. This allows for the website to see exactly how you did on your PSAT, and then it recommends problems sets for you to study, based on what you missed. You are also able to set up a study plan on the website, with what date you plan on taking the SAT entered in, so you can set up a study plan that will help you learn what you need by the date you need. You can set up what days and times work best for you, and it will send you reminders to keep your study sessions. It is essential to come up with a study plan that works for you, because you are going to get so much more out a consistent study practice rather than trying to cram in a a bunch a few weekends before the exam.
If you are looking for an Math SAT study book recommendation, I like Pwn the SAT by Mike McClenathan. The book is mostly geared towards people with scores in the 600s, wanting to make them higher, but I think that anybody could make use of this book. What I like most about this book is, in the first few pages, it gives an excellent template for how to study for the SAT, both if you have a lot of time, or only a little time left before you take it. The book also has a mixture of practicing different content sections, along with different strategies for how to take the test. It approaches the SAT as a unique math test, because, well, it is a unique math test! It is unlike any other test you have taken in math, and thus, it's worth thinking about other ways to approach math problems, than our usual techiniques.