Depends on your level of {experience, intended engagement, patience for pedantics, intended outcomes}. I've found that 3Blue1Brown (youtube) gives a great star

Depends on your level of {experience, intended engagement, patience for pedantics, intended outcomes}. I've found that 3Blue1Brown (youtube) gives a great starting point for many topics. Provided you actively participate in them, the videos will build up some intuition so you know what questions to ask (and why they're worth asking). The author usually links to followup resources so you know where to go next. I've found his resources a bit hit-or-miss, but they give me enough to know if I want to continue exploring. If I do, I usually check out a good textbook (I refer to https://news.ycombinator.com/item?id=17617825 https://news.ycombinator.com/item?id=17617825 often).