Once we're outside a formal academic setting, the way to start learning is to start learning.

The first high hurdle is accepting that starting out everything (to a first approximation) is over our heads.

There's no perfect first resource because hard subjects are hard and take time.

But because we are out of school, we have decades to learn.

There's no final in sixteen weeks and only a pop-quiz tomorrow if we are in the middle of applying what we learned.

So just start learning math and figure out what works for you as you go along.

Good luck.