Hey, Khaoula! You can say: "I wanna be a hundred percent sure about it". I hope this helps!

Hello. The sentence you wrote sounds a little off. A more natural way might be "I want to think about it/things a little more". hope this helps.

Or as a variant, you could say, " Let me think this through."

"I wanna" only makes sense if you speak in a slurred, lazy way. You would not write it like that; use "I want to" in normal, everyday writing. I'd write, "I want to get it clear in my head".