Hi there, I am Superman. - should be with a capital letter, referring to Superman we know from movies. With this sentence you imagine you ARE in fact the same person as Superman. Maybe you put on a costume at Halloween and say this sentence:) 2. I am a superman. - It means that you compare your qualities to those of Superman and you can do things like he does. Maybe you say this sentence after you saved someone in a less serious situation. I hope this helps.

Yes both answers are correct. 1. I am Superman. - Meaning that you are literally Superman. 2. I am a Superman. - Meaning that you are like Superman but not 'The Superman'.