First of all, you would be remiss if you did not consult your high school office to see what resources your school might offer. Often schools will offer special SAT practice exams or diagnostic exams called the PSAT. This can give you a great feel as to how you are positioned in relation to other test takers and high schoolers. Your school may also be able to point you in the direction of reputable tutors or materials that can help you to study for your standardized tests. Furthermore you may find that you are eligible for some form of test fee discount through your school or through a quick application to the College Board. There are a lot of resources that your high school counseling office may be able to offer you.
There are hundreds if not thousands of books for the SAT alone promising readers that through following the instructions in their books, you will test into the top percentiles and be the envy of your college minded peers. While that may be true, the best way to prepare for a standardized test is to take tons of past tests or official practice tests. To this end I would recommend purchasing one of the College Boards official compendiums of past tests or mock tests. Why do I suggest this? Taking a practice SAT will tell you what sort of material you will find on the test including the types of problems or vocabulary you will see. More often than not, you also see that the College Board recycles old problems from the SAT. While getting the Princeton Review testing editions is nice, you cannot beat stuff straight from the source.
In conclusion, there are loads of resources that are available to students and you do not necessarily need to buy an expensive tutor or testing regimen. Through your own private research and inquiries, you can find modestly priced options to help you achieve the score you deserve.
-- Gabriel Moran