In this case, there are two possible answers. 'Am I?' is grammatically correct but a bit unexpected in isolation, as here; 'Aren't I' is acceptable spoken English and probably the more common tag; it should be replaced, however, by 'am I not?' in formal English.
You might rephrase your sentence as follows. "I am your friend, correct?" Or you could say, "Am I correct in thinking that I am your friend?" Or, "Am I correct in thinking that you are my friend?"