It's I !!

Good day!
...So, what's the different between "It's me" and "It's I"?
And which one would you say when you look into the mirror and realized that it's yourself staring back at you?
Thanks for your help.

edit-edit-edit: follow up questions:
can "it is I" be used to emphasize the "I"?
was "it is I" used in ancient time?

um, i think i should explain that i might want to use "it is i" in my fictional work... but if it's far too grammatically wrong, then i won't.