If this is a hobby, seek help! But, seriously…
I’d start with a book on the subject of College Algebra. This will teach you many concepts which are applied in Calculus while refreshing your advanced algebraic skills. Next, you should get a Trig book to brush up on working with trig functions, vectors, and such aspects of trig. This shouldn’t take you too long to get re-acquainted. Lastly, find a book on Linear Algebra to get you familiar with the ins and outs of working working with matricies and determinants. This isn’t hard, either, really. Most of it is just algebra. (Except some upper-level concepts can get a little abstract…Eigen values? Kernel? Ug!)
After this, you’ll want to get a Calc book and start digging in. Some calculus gets kinda hairy, but most of it you should be able to self-teach without trouble. Some complex integrals (at the Calc II level) get quite ugly because they require tricky approaches outside the norm.
higher level calculus gets into 3-D problems. This isn’t too bad, and if you’ve mastered the matricies of the Linear Algebra I mentioned, this won’t be too hard.
Lastly, depending on how much calc you like, you can even teach yourself Ordinary Differential Equations (ODE) often known as “Diffy Q”. This is where your algebra teacher was correct - you’ll need to use that confounded quadratic equation over and over. And then, if you master this, YOU can teach ME Non-linear (or non-ordinary) Diffy Q!
I don’t know group theory…is this about Venn diagrams?
If you’ve mastered all this, then you can be like my college roommate: a 4.0 Physics major (now Ph.D. in astrophysics) who was proving the integrals tables AND deriving solutions for integrals beyond this!!!
Quoting my profs’ understatement of the past century, as they’d beat this mantra into us: “It’s just basic algebra, what don’t you understand!!!” (Yeah, right buddy!) - Jinx