The first place to start for test preparation is with the information provided for you by the testing agencies on their websites. Both websites, ACT and SAT provide valuable information describing the test, strategies on test taking, practice tests, etc. They have self-help books available to purchase as well as online tutorials to purchase.
Getting familiar with the structure and subjects of standardized tests can help you score high. At http://www.petersons.com/college-search/test-prep-topics.aspx, you'll find information about the most common standardized tests for students going to college: SAT, ACT, PSAT, AP exams, GED test, etc. You'll also be able to take practice tests on the ACT, SAT, GED, etc...
There are numerous companies that offer test preparation classes and tutorials. The website, Reviews.com has a section that compares the “SAT/ACT Test Preparation Companies” available; rates & reviews them, and provides links to each company’s website:
Princeton Review ACT/SAT
Set for College
Top Test Prep