When you're "into" something, it means that you like it. So when you don't like something, you can say that you're "not into" it:
I've never been into baseball.
You should ask her out. She's totally into you.
A: You want to smoke some weed?
B: No, I'm not into that sort of thing.
"Into" and "not into" are casual phrases. They've become popular pretty recently, but they're not too slangy.