honestly, just take tests haha. it may seem aimless, but there is straight up very little you can gain from doing anything but practice tests.
Make sure you've taken through Algebra-2/Pre-cal or whatever you need to answer all the questions on the math section.
Buy a princeton review or something and read every single rule from the english section (commas usage, colons, conjunctions and all that random stuff). Make sure you know all the english grammar rules.
And then from there on out, just take a practice test weekly or biweekly. make sure you go over the answers you missed after the practice test. you will see your score increasing, and you will get faster and faster everytime.
also, try and make sure you only take REAL ACT practice tests. princeton review, barrons, and all those other tests weren't made by the ACT and thus have inaccurate difficulty levels.