The thing that’s most important when preparing for the ACT is learning how to manage your time properly. The material/content on the ACT is not bad whatsoever, but the thing that gets almost everyone is how fast-paced the test is. There are numerous timers online made for this purpose which can really help you out with that aspect of the test. Also, when you register for the ACT there’s an option to buy a subscription of I believe it’s the Kaplan(?) ACT Online Prep? It’s really expensive, but I got a free six month subscription with an ACT fee waiver and it actually helped me a lot with the grammar concepts.
What do you mean by “past papers”? Are you referring to practice exams? There are many online, although the official ones are the most reliable. Check out this PrepScholar blog post. They have links to previously released official exams.