In the college consulting field, we have an all-star neighbor here on the north shore of Long Island: Andy Lockwood of Lockwood College Prep. His company is offering a fantastically useful free webinar this evening at 8 pm, sharing analysis, tips, and tricks for approaching the ACT, and on differences between the SAT and the ACT; you can register at this link.
I agree completely with Mr. Lockwood's assertion that the difference between a good score and great score on any standardized test "is frequently about strategies and tactics, not what the student learned in school." With that observation in mind, a student should never approach a standardized test cold. Do these three things instead:
Create an account with Khan Academy's SAT practice site, which allows for students' College Board accounts to be linked--therefore, their PSAT or previous SAT data personalizes the practice questions they are given.
Students should also plan--on their own or with a responsible friend or adult--take at least one timed practice test on paper under simulated testing conditions.
If you're still deciding whether the SAT or the ACT is the best fit for you (in terms of your test-taking preferences and strengths) here are two incredibly helpful and insightful infographics explaining the differences, one from Princeton Review, the other from magoosh.com.
Most important: relax! At the end of the day, any standardized test result is, at best, a poor and partial accounting of a particular performance on a particular day. Recognizing the essential unfairness of standardized testing, over 1,000 colleges have adopted test-optional admissions policies; a list of those institutions can be found here.