think for (oneself)
To have opinions or make decisions without letting other people dictate to or influence oneself. You can't just blindly follow what your boss says, especially if you think he's unscrupulous—you need to think for yourself! Hannah needs to learn how to think for herself, or else she'll just follow whatever her friends are doing.
think for
oneself to do one's own thinking; to think independently. I think for myself. I don't need anyone to tell me what to do, do I? Sam has to learn to think for himself. He can't let other people make his decisions for him all his life.