The SAT is often hyped up to be the deciding factor in college admissions. It is important to remember that the SAT does nothing to prove the three key factors (Passion, Initiative, and Authenticity). The test provides a snapshot view of your academic potential and abilities. Doing poorly on the SAT or ACT is damaging to an application but does not mean you have lost your chances. Students with low SAT scores are accepted all the time to Ivy Leagues and other top universities.
The ACT is considered to be a more straightforward test that is based more heavily on school curriculum than on critical thinking skills. You should take both tests to see which score is the preferable one. Some students feel the need to take the test many times, but this is a futile strategy. It is better to take the test two or three times and move on to focus on other parts of your application. An admissions officer at Harvard once joked with a group of visitors that they consider the SAT an extracurricular activity after you have taken it more than four times. In order to sign up and pay for the exam, you must visit the respective site to create an account. You have to bring a photo ID and some pencils the day of the test.
You should make sure you take the Essay portion of the ACT, as most colleges require it. The Essay Section helps provide the admissions committee with a sample of your impromptu writing. Both the ACT and SAT make the essay available for the committee to see.