I'm not a native speaker, and I'm out of practice with Korean. But this is how I would translate these sentences. I don't think I've heard of it: 지금 처음 들은 거 같은데. (I think it's the first time I've heard of it.) I don't think you'll have heard of it: 네가 이거 모를 거 같은데 (I don't think you'll know this.) If you want to say "I think" in general, you can usually use ㄴ 거 같다 or -다고 생각하다. I gave examples of the first one, so I'll give an example of the second one. Be warned though that I wrote this one, so it could be wrong. 자기하고 가족의 행복을 위해 이거 안 했으면 좋겠다고 생각하는데요. I think it would be better if he didn't do this for the sake of his own and his family's happiness. (I was talking about a guy who is obsessed with an anime character and spends a bunch of money on figurines of her and has a body pillow of her that he takes on dates.)

Just a general note.

Korean tends to use grammar over words/vocabulary to express meaning. Therefore, in different situations, certain concepts (such as "I think") could be expressed in various ways, and it's more about grammar and the situation than direct translation. Ta-da.