you can use khan academy. I believe they give out free lectures and practice problems.
if you haven’t learned calculus start with that and go onto calculus 2 after. then learn multivariable calculus and linear algebra, and then differential equations. once you do that you’re at a level where you can start learning undergraduate stuff, for which most people recommend looking at the springer “undergraduate texts in mathematics” book series. I can’t say anything about it though since I’m not at that level, but you can probably find the books themselves for free on libgen.is