For learning ACT grammar rules, I highly recommend reading Erica Meltzer’s Complete Guide to ACT English (which has lessons as well as practice questions and actual ACT tests). For Math, PrepScholar co-founder Allen Cheng has recommended Ultimate Guide to the ACT Math, by Richard Corn. If you're scoring 16 or below on Math practice tests, you might also find the free Khan Academy math modules useful in teaching basic concepts (particularly if your school doesn't cover them).
There's also a bunch of high quality free resources available on the PrepScholar blog. I’d recommend reading through the ACT Math and English articles on our blog, as well as the more targeted ACT grammar rules article. These articles break down each subject area into discrete skills that you can then train. Finally, our article on alternatives to ACT practice test lists additional resources that might help you with firming up your content knowledge for ACT English and Math.