Ask your teacher for suggestions. Many textbook companies provide supplemental materials that include standardized test practice. Your teacher might also have practice materials that she has accumulated over the years.
Look through your textbooks for exercises designated as standardized test practice. The exercises may be scattered throughout the text or may be included in a separate section.
Check with the guidance counselor at your school. Since he is usually the person responsible for organizing the registration process, he may also have access to study and practice materials. At the very least, he should be able to recommend good practice resources.
Use the practice booklet issued to you when you register for the exam. This booklet contains practice questions that are representative of the types of exercises on the actual test.
Use online sources. There are several good online sites that provide free practice for both the ACT and SAT. Most include answers and explanations to all the exercises and some even include an excellent review of topics covered on the exams. The practice tests can be printed out and taken under simulated test conditions if desired. See the resources below for links to some sites that provide free practice.