Oh yeah, and use tools like WolframAlpha to check your work and explore problems in more depth (like visualizing functions, seeing alternate forms). Get the app and use the solution explainer thing—essential for e.g. solving integrals.
It might be worthwhile to learn to solve and verify problems using some sort of (mathematical) programming language or a CAS. Could be anything, but something like SageMath comes to mind. Honestly, even Excel is pretty good for this. Being able to do simple things plot functions, verify work by plugging in values, simulate random numbers, etc. goes a long way. Developing this skills becomes even more useful (essential) when you’re at the college level and beyond.
First of all, good on you for going back to school—that’s not always an easy thing to do. What subject are you considering studying?
Khan Academy is definitely the best one-stop-shop for this purpose, as someone mentioned. The key is to consistently do lots of practice problems over time. KA is adaptive+gamified, and helps select problems in your “zone of proximal development” (not too easy, not too hard).
Depending on what you want to get a degree in, you can focus on particular areas and gloss over others. If CS, then discrete mathematics and logic are the most important (plus stats/probability/linear algebra for machine learning and AI). If engineering, then trigonometry/calculus/physics is more important.
Learning also requires motivation, which for me personally requires seeing the big picture of why the math theory matters and how it was developed. Read the NYTimes series by Steven Strogatz. I also like “Mathematics for the Nonmathematician” for an overview of HS-level math that weaves in historical context (e.g. how Renaissance art or agriculture and mathematics are intertwined). Watch some YouTube videos on mathy subjects (like Numberphile or 3Blue1Brown or any of the zillions of channels along these lines). I try to do this as a “learning by osmosis” sort of activity that I fit in to my daily routine, e.g. when folding laundry or commuting.
Learning is also a social activity, so maybe enroll in a community college course or find a local study group. I find it’s especially important to have someone to discuss things with when learning math. I also recommend finding good public spaces to work in—libraries and coffee shops are timeless math spaces.
Along those lines—as you’re learning/reading/practicing new concepts, imagine explaining them to someone else (Feynman method).
Do lots of problems by hand on pen and paper, there is research and eons of practical experience that shows that doing math is a kinesthetic experience (that is, there’s literally “muscle memory” for math). Draw pictures and graphs on paper. Keep all this scratchwork and doodles and stuff in a notebook.
Learn to process the “I have no idea what’s going on” thoughts and feelings you get when you’re faced with something new and challenging, or seem to continually forget things you’ve just (re)learned. That’s par for the course, you just have to “feel the burn” and keep going. Cheesy but extremely true.
Here is a great video (and a great channel generally) on the subject: https://youtu.be/MRipzKkeQik