How do I get better at ACT-level geometry?

Khan Academy needs to become your best-friend. Rather than remembering formulas and useless algorithmic thinking, learn to derive the formulas and truly understand it in your own way via intuition. Practice every day quite often. Even if you aren't a big fan of math, this is necessary. Remember that fundamental mathematics (e.g. pre-algebra, algebra I, algebra II, geometry, trigonometry, calculus) is fairly sequential and each tier up generally expects you to have a good understanding the previous subjects. So, if you're missing fundamentals like the Pythagorean Theorem, then you need to get the foundations like that done before you move to 3D shapes or anything more complex. Try to find how you learn best and utilize it. Maybe geometric constructions aren't your thing, well go online and find some algebraic math to go with your geometry and make it easier or more intuitive for you. Make it make sense to you. Maybe your teacher recommend technique X, but frankly, if techniques Y or Z work better for you then use them! If you get stuck on a problem for too long then use a site like this one or stackexchange to help you (this is talking about in the test-prep phase of course). Make sites like Wikipedia your friend, because, albeit not acceptable as a citation, it is a great learning tool (and most pages have very reputable citations near the bottom). Don't be afraid to ask for help in whatever way you need. This extends to in the non-virtual world too. Ask a friend, a mentor, a parent, a sibling, etc. Read some information on the topic via book. You may be able to find a basic geometry book at your local library or a cheap one at a book store, just use that. Or, you can search the web and/or use torrents to look for a book of sorts. Practice ACT example questions and similar ACT exercises. I hope this will help, best of luck! (comment or message for any follow-ups)

I categorize ACT math into 5 general topic areas and 5 levels for each area. If you take any math section of the ACT you can roughly think as the first 12 questions as Level 1, the next 12 as Level 2, and so on up through Level 5. Note that this really is a rough estimate - although the ACT sort of increases in difficulty from the beginning to the end of each section, the way it increases is very "wavy." But if you don't have a person or a prep book that offers guidance, you can use this rough estimate. The specific topic area you are concerned with is Geometry. So first figure out the Level you are stuck at. Let's assume it's Level 1. Then practice only Level 1 questions until you become somewhat proficient in them. Then move up to Level 2. I don't like to worry too much about specific Geometry topics. In my experience it's much more efficient to tackle the overall subject by difficulty level of the problems. This way you will naturally review only the concepts that are likely to improve your score as you prepare. Note that this method applies to the other 4 subject areas as well (Number Theory, Algebra and Functions, Probability and Statistics, and Trigonometry). Your level may be higher in these, but you should still figure out what hat level is, and work on getting to the next level.