Buy / rent / borrow at your local library any ACT prep book and start following that. They will typically ask you to start by taking a practice test of the ACT to see where your weak spots are. Then you study those sections the most, take frequent practice questions, analyze how to do better (e.g. is it a timing issue, a lack of knowledge, misreading questions, etc.), and then you take another practice ACT test again to see where you've improved and where you still need more work.