You just learn your math, language skills. Purpose of test is to show you have learned properly. A book about SAT can show you what is on test, generally, but you have to learn mostly. Best way to prepare is to read for pleasure, honestly.
Try to get an SAT practice book. McGraw Hill, and Kaplan are good ones. Then do practice tests daily. Of course, you'd learn the skills you need more than likely in highschool, but the practice really helps.