Preparing for the ACT and SAT is really important in order to get a good score. The following resources are great for accessing free test prep for the ACT and SAT.
Khan Academy: Khan Academy is an online resource that is a great resource for practicing for the SAT. Khan Academy can offer a personalized study plan for the SAT based on your strengths and weaknesses.
This graphic shows the average number of points gained from PSAT/NMSQT to SAT associated with the hours spent practicing on Khan Academy.