Sample questions for the ACT are available online at the ACT's official website at ACTStudent.org. It contains free sample sets of questions for English, math, reading and science. Additionally, it offers a sample writing prompt along with responses.
Another online source of sample ACT questions is 4Tests.com. This site is also free and includes samples of all the ACT subjects except writing. However, solid academic preparation throughout high school is the best way for a student to prepare for the ACT. This includes completion of challenging coursework in English, math and science, all of which prepare a student to do well on the ACT.