I don't know or i do not know?

Same meaning, But, people normally use "I don't know" to make it simple/short in a conversation. In written language it is more formal to use "I do not know".

Usually people say I don't know. I do not know if when you want to assure somebody 100 percent that you do not know or when you are typing a sentence like I just did then it sounds ok too. I do not know when you will be home while writing sounds good while speaking you would say I don't know when youll be home Both are find though and I wouldnt make a big deal about it, but you would sound more like a native if you followed these rules

Both are correct. "Don't" is just the contraction for "do not".