Below are only a few of the available online practice sites.  However, keep in mind the ABSOLUTE BEST way to prepare for the ACT/SAT is a prep course that is taken in person and uses daily pratice throughout the year.  Neville offers such courses for free--talk to your guidance counselor about available courses. 

Princeton Review Site: http://www.princetonreview.com/college/free-act-practice-test.aspx
ACT Resources (Official site): http://www.actstudent.org/testprep/

Unofficial Practice Site: http://www.4tests.com/exams/examdetail.asp?eid=13