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.
Finally, learn in advance how far a little deductive reasoning and some skillful time management can take you in the direction a great score--either by registering for and tuning in to Lockwood College Prep's free webinar tonight, by reviewing Fastweb's list of standardized test-taking tips.
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.