If you are looking for books, check out this site: http://www.test-study-guides.com/best-act-books/
Here's a free site: Number2.com :: Free Online Test Prep
Check out Khan's Academy. I believe they also offer free SAT and ACT prep.
I personally do NOT recommend the Barrons series for test prep. I've never done well on standardized tests, and at least for me, Barrons only added to the confusion.
My question for you is this: have you taken at least Algebra 1? Whether you take the SAT or ACT, you will need a minimum of Algebra 1 since they both test algebra. Or, if you're terrible at algebra (I am), I'd recommend https://www.amazon.com/Beginning-Algebra-11th-Margaret-Lial/dp/0321673484/ref=sr_1_1?s=books&ie=UTF8&qid=1468950621&sr=1…
This is a Beginning Algebra book used in developmental math classes in college (you don't receive college credit for developmental math), but it is the equivalent of Algebra 1. The authors here make algebra more accessible and provide step by step instruction. When I used this book, I actually understood algebra for the first time in my life.