I want to know which sentence is correct. If i say
Guess, what it is?
Guess, what is it?
which one is correct?
Guess, what is it ?
I remove the punctuation for us to get all the possibilities and study each as I think this is the appropriate way to answer this question:
Guess what is it/ it is
The person might have ordered the audience; as if s/he is a teacher asking a question:
Guess what it is.
There's only one sentence and it's imperative.
The speaker is apparently very emotionally intrigued. S/he wants to tell the audience about a piece of news that has exclaimed her.
Guess what it is!
On the other hand, this might come unusual to a native speaker as they usually omit (it is) and get:
Note that this is mainly a question, but since it's turned into an expression and it's exclaiming connotation is usually "heavier" than it's questioning connotation, we often see it with an exclamation mark.
As Edwin mentions, a correct punctuation plays an important role on deciding what this statement means.
Guess, (or you can just put a dot or a colon) what is it?
As the title of the possibility describes, guess can act as another imperative sentence that has no relation to the following one. "What is it?" is the correct structure if you're asking a question.
So your first statement has to be "repunctuated". Otherwise, it can completely apply to possibility one.