The ACT and SAT are two different tests that can be used as a part of your college admissions materials.  You may use one, the other or both.  
JUNIOR year is a big year for testing.  Holly Springs High School gives every student the ACT test in the Spring of their Junior year.  

After you take the ACT in March you may want to improve your score on the test or may want to try the SAT.  You can go directly to their websites and sign up for additional test dates.  

Preparing for exams is an important part of this process.  Fortunately, high-quality test prep resources are increasingly available for free online. The College Board, which provides the SAT, partnered with Khan Academy to offer free personalized SAT test prep online. Khan Academy can help your child create a personalized SAT study plan targeting the areas your child needs to work on that includes video lessons, practice questions, and full-length SAT practice tests. Students can even use the service to study on the go by downloading an app to their smartphones or tablets.  The SAT also offers a free Student Guide and sample questions. 

ACT offers free resources including a study guide and sample questions, which students can access by creating an ACT Profile or reviewing free questions and content descriptions for each ACT section: English, math, reading, science, and writing. More free online practice — including full ACT and SAT practice tests — is available through Kaplan Test Prep.
You can also check to see if there are any free test prep classes or proctored practice tests in your community. Many libraries, universities, community colleges, and high schools offer free or inexpensive test prep programs. You can learn about local opportunities from your student’s high school guidance counselor or at the local library.