The PSAT gives you an idea of how you will perform on the SAT. The test is given through your school so ask your guidance counselor about it. At my school, kids take it from their freshman year to junior year, but people generally take it their sophomore and/or junior years. There is a scholarship program called the National Merit Scholarship Corporation that is based on how you score on the PSAT. However, be aware that the requirements are high- only the top scorers in the country, and then there are semifinal and final rounds before scholarship winners are picked. Here’s some more information on the program if you’re interested.
As for the SAT, I have found that the best time to take it is during the second half of your junior year and/or very early on in your senior year. What grade are you in? The summer is a great time to take it, but if you’re already a junior I’m afraid that ship has sailed since the last test offered before the next school year was in June. If you’re entering your senior year in the fall, I recommend registering for the earliest test dates offered as soon as registration becomes available. Give yourself as much room as possible to retake the test before college applications if you’re not happy with your scores.
SAT Subject Tests are just what they sound like- standardized tests in specific subjects. Some schools will recommend or require that you take a certain number, while other schools don’t ask for them at all. Look at what schools that you are interested in say about them. If they don’t recommend or require them, then don’t bother with taking the tests, they will just be a waste of your money. If any of the schools do require them, the good news is that you can pick any subject(s) that you want. Pick subjects that you are very knowledgeable in and that will show your mastery of these subjects. If you’re unsure of which tests you’ll do best on, there are practice tests for each subject available on the College Board website so you can get a feel for what you should take. I will say that just because you do well in the class and on AP tests for a subject does not mean you will do well on these tests, so I highly recommend the practices. For example, I did extremely well in AP Calculus, and AP U.S. History, but not quite as well in AP Chemistry. However, I did best on the Chemistry and Math 2 subject tests, and I did somewhat poorly on the U.S. History test. (Just some advice, I have heard similar results from other people about the U.S. History subject test, so I definitely recommend taking the practices for it if you are considering that test.)
Hope all of this helps! Good luck with the whole process!