“Learning how to think” really means learning how to exercise some control over how and what you think.
It means being conscious and aware enough to choose what you pay attention to and to choose how you construct meaning from experience.
Because if you cannot or will not exercise this kind of choice in adult life, you will be totally hosed.
Think of the old cliché about the mind being “an excellent servant but a terrible master.”
This, like many clichés, so lame and banal on the surface, actually expresses a great and terrible truth.