"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?