The best way to prepare for the ACT is to take full length practice tests. Students should review the answers they got right AND wrong. If you find that you are running out of time on a particular section (usually Reading) then you will need to practice that particular section several times by taking timed full length tests.
Studying should start at least 12-13 weeks BEFORE the ACT test date.
The top four reasons for taking official ACT practice tests are to:
get accustomed to the experience of sitting down and taking the real thing (including becoming comfortable with taking the full test at 8am)
build up your stamina and ability to focus for long periods of time
figure out where your weaknesses are
judge your progress and studying efficacy
Identify your strengths and weaknesses. Review whatever content is needed.