Updated: Mar 2, 2018
There are a lot of great test prep books and private courses, and I am not discouraging them. Just make sure that your study program emphasizes reviews from the actual authors. Their practice tests and reviews will always be the closest reflection of what is on the actual test.
The College Board writes the PSATs and the SAT: www.collageboard.com
For the ACT, use Peterson's Real ACT Study Guide: www.actstudent.org
After you have mastered the content from the authors of the test, then you should branch out to other test preparation companies (books, classes, etc.).
Don't forget about the many great resources that are available for free on the internet: YouTube and Kahn Academy.
Tip: Don't ever feel like you have to spend a lot of money. If you know how and where to look, you can create your own highly effective SAT study program.
Watch the SAT and ACT videos to find out how you can go to the source for your practice tests. Not all practice tests are created equal; you want ones written by the original authors of the tests.