Prepping for the ACT exam doesn't have to be difficult especially if you start early. But all "prep" is not created equal. There are many test prep books available on Amazon and in bookstores. You may even find some at used book sales. But use caution. Both the ACT and SAT (College Board) spent a ton of money getting those test questions just right. There are a lot of copycat test prep books that may not be substantial enough to be used for your prep work.
Official practice tests are the best way to go. ACT and SAT both retire several exams each year. Those exams are usually placed in their official test prep books and are available for purchase each year.
ACT offers an official practice test that you can print out and take with a pencil and paper. I highly recommend doing this since your real exam will also be using pencil and paper. Fo any practice test you'll want to mimic an actual testing situation as closely as possible. with timing, and no distractions, etc.
This same exam has an online version here. You'll have to have a MyACT account: https://www.act.org/content/act/en/products-and-services/the-act/test-preparation/free-act-test-prep.html
Once you've determined the type of questions you need help with, you can use ACT's practice questions bank to hone your skills. If you are starting early as I recommend (middle school) you should be building your own personal test bank.
Another test prep company partnered with the ACT and provides a 1/2 length ACT exam, a short quiz, and a question of the day. https://www.kaptest.com/act/free/act-free-practice-test
Keep a notebook to keep track of the types of questions your teen got wrong. This is not necessary until your teen has at least completed Algebra 1 and is an excellent reader. Keeping notes to know what still needs to be learned and what rules to memorize is an important part of test prep. Don't just take the exam. Learn it.
If your teen needs personalized help with the ACT or SAT exam prep, my recommendation is PrepExpert. They have options from DIY courses to 1:1. Throughout the year they offer discounts on their programs. https://shareasale.com/r.cfm?b=1534521&u=2570158&m=97094&urllink&afftrack&fbclid=IwAR05grvQ8zZOr2245WvZEkItOsWl6BfMmJ7I1PyI8tjb-MzuU4n5PlcYjlg