How do you learn new things? (How do you pick what to learn? How do you start? How do you stay motivated?)

There's just too many things I'd want to learn, but I probably can't do everything at once. And I also don't know where to start when it comes to basically any topic.

Courses could absolutely work, but they tend to be paid, so that's a problem.

How do you approach learning new material, especially if you only know a goal rather than a full/partial path?