>>14244831Learn set theory, how to do proofs with sets.
Get good at trigonometry.
Get a fat calculus textbook with lots of problems. This site is good for calculus:
https://tutorial.math.lamar.edu/ - however its not a substitute for proper textbook, as the site's author himself has notes.
Learn linear algebra. I used Linear Algebra - A Modern Introduction by David Poole(2nd ed), very beginner friendly, lots of proof problems. Linear algebra is a good introduction to proofs.
Learn the theorems by heart whatever you're studying, and understand the proofs. Do not gloss over the theoretical stuff, wherever you see it.
Check out Discrete Mathematics. It's like an introductory survey of college-level, theorem-centric, mathematical topics. It teaches a lot of basics that apply everywhere- formal logic, sets, functions, proof techniques, combinatorics(formalized counting). I used Rosen's textbook(4th ed), which included a lot of interesting material in the problem sets, and the problems were fun.