"who is there?" or "whom is there?" I hear people use both, which is more correct? Can we interchange both words as we like it?
Only "Who is there?" is correct. See explanation.
Who is a question about subject of a sentence, while whom is a question about object.
For example if you have a sentence like: "John is at school now." you can ask a question starting with who: "Who is at school (now)?" This sentence does not have an object, so you cannot write a question using "whom".
But if you consider asentence like: "Ann saw Betty at school.". You can see that this sentence has both subject (Ann), and object (Betty). This is why you can ask questions about subject
*Who saw Betty at school?*
but also about object:
*Whom did Ann see at school?*