I will say from own personal experience with the ACT. First and foremost, I would say take a practice test and identify your greatest weakness, mine was math. I bought the Official ACT prep book with five tests in it and I sat down and took the entire test simulating testing conditions then scrutinized my incorrect answers and asked my math teacher at school for help when I did not understand why I got them wrong. I also downloaded older tests off the internet that were free (there are literally dozens) for more practice; also focus on pacing time will get you on the ACT. After my first official ACT test I got a composite 22, I'm kind of guessing what I got because it was super long ago, (English 17, Math 19, Science 20, Reading 29). Then I retook without any practice and got a 25, once again kind of guessing, (English 20, Math 22, Reading 29, Science I don't really remember, but something low). After this test WP told me I was a RISK candidate and I needed to receive a minimum of a 27 in all categories to have my file continued, so I studied like crazy via the method I mentioned for a month and during the week leading up to my third ACT I used Kaplan advanced math prep course, which is like an hour and a half of a lecture showing harder math topics on the ACT which helped tremendously. I received a 30 on my third ACT (english 31, math 27, reading 32, science 29) with like an 11 on the essay (essay is a joke it's so easy).
Moral of the story, commit to a plan and you can achieve your goals with grit and determination.