The ACT is a curriculum-based test, so it covers what you learn in school. To take full advantage of your education, ACT recommends that you take the test in the spring of your junior year. This way, you'll have time to re-take the test, if you choose, before college application deadlines.
Free test prep aids are available at www.actstudent.org. Sample questions are on the site along with Preparing for the ACT, a free booklet with a complete practice test, scoring key, writing prompt, and sample essays. The ACT registration packet, available from the counseling office, also contains the booklet.
ACT Online Prep, a test prep program developed by ACT test development professionals, is $19.95 for a one-year subscription. The Real ACT Prep Guide, a book featuring retired test questions, test-taking strategies, and optional writing test overview, is $25. Both Online Prep and the Guide are available at www.actstudent.org.
Source: ACT's News You Can Use