These days, you use Google. I personally have a list of books for intro physics [0] and help maintain a meta-list of books up to graduate level [1], and there are hundreds more lists out there.

Heck, when I was a kid I learned from just checking out books at random from the local public library, and even that was fine. There's just an avalanche of great free resources out there.

0: https://knzhou.github.io/#writing 1: https://physics.stackexchange.com/questions/12175/book-recom...