Definitely use number 2. "I am not really into it."
Sentence one is an emphasis on you have no interest in it. Definition b: truly, unquestionably
- I am really not into it = I am truly not into it.
Sentence two is a statement that you don't know much about it or you don't use it. The definition is "in reality, actually". So "I am not really into it" means "In reality, I am not into it".