"저는 누가 입니다?" is not quite right.
The base word for "who" or "whom" is 누구.
When it happens to be the subject of a sentence, it contracts to 누가: 누구 + 가 (subject marker) -> 누구가 -> 누가.
When it's an object or a subject complement as in your example, 누구 is the correct form.
Your sentence is strange even after the correction because 저는 누구입니다 ("I am who.") has the statement ending 입니다. You should use the question ending 입니까?
Here are examples of correct sentences:
- 저 사람은 누구예요? = Who is that person?
- 나는 누구입니까? = Who am I?
- 이 책은 누구를 드릴까요? = Who should I give this book to?
- 이 책은 누구(의) 것이에요? = Whose book is this?