I was going to think about that later so I don't know yet.
I haven't thought about it yet.
1) I was going to think about that later so I don't know yet.
それ後で考えようと思ってたからまだ分かんない。
〜しようと思ってた は I was thinking of 〜 や I was going to 〜 と言います。
まだ は、文章の最後に yet と加えます。
I haven't eaten yet. まだ食べてない。
I haven't read that book yet. その本はまだ読んでない。
2) I haven't thought about it yet.
それはまだ考えてない。
I'll decide later (後で決める) などとも言えます。