Peruse your local bookstore or your favorite online bookseller for ACT (American College Testing) preparation books. Choose one that not only has detailed information about the subject matter of the test, but also has practice tests.
Start studying early so that you know where you're strong and where you need to brush up. This will also increase your confidence and save you from stressing about the test.
Work through the book, little by little every day. Make studying for the ACT a part of your daily routine.
Take as many practice tests as time and patience will allow. Simulate exam conditions as best you can; for instance, make sure you have a 3-hour chunk of time when no one will bother you.
Talk to people who've taken the test. Battle-tested veterans often have the best advice.