Start with linear algebra and (multivariable) calculus. Universities usually teach these in the beginning semesters. I would argue that linear algebra is the most important one (see also lecture from Princeton).

- Linear Algebra Full Course - Dr. Valerie Hower:

https://www.youtube.com/playlist?list=PLpcwHaLYiaEXW5fLNOlIt...

- Multivariable calculus, full semester - Diana Davis:

https://www.youtube.com/playlist?list=PL3e7J-yDTmahubJZoHodT...

- Linear Algebra (Princeton University)

https://www.youtube.com/playlist?list=PLGqzsq0erqU7w7ZrTZ-pW...

- immersive linear algebra

http://immersivemath.com/ila/index.html

- Linear Algebra Done Wrong by Sergei Treil

https://www.math.brown.edu/streil/papers/LADW/LADW.html

Axler's book on linear algebra seems to be well written and concise.

I have more, but, I am multilingual so, I guess they can't fit in here.