Pick up the free student preparation booklet with your registration packet from the guidance office. It contains test information, complete practice tests with scoring keys, and a writing prompt with sample essays. If your school does not have the booklets, you can go to http://www.actstudent.org/testprep/ for a copy.
Check out an ACT prep book from your school or local library. A good book to use is The Real ACT Prep Guide. It comes with five retired practice tests, detailed answers to each question, and scoring guides for each test.