Where to start learning mathematics? I want to find a starting point to learn "formal" math. While I've always been interested in math and am good at high school math I know hardly anything beyond that.
Some YouTube channels, like 3blue1brown, give you intuition for some phenomena I still don't know how I could start learning some rigorous math before going to university.

Historically, a lot of people have started with number theory or graph theory.  They are fields with accessible concepts and interesting questions, and they'll get you into the practice of formal proofs.
My professors taught me that if I wanted to learn X theory, I should just walk down two floors to the science library and pull any book off the shelf called "Introductory X Theory", because they would all say pretty much the same thing.  I must say that this rule has not let me down very often.  You could do the same thing with Amazon.  The cheapest used paperback book on a topic was reprinted in paperback because it was good enough to be read casually.
