Usually it'll be "It is me" though it'll be contracted to "It's me". Example: Sarah: "Who is it?" John: "It's me." Also I did a Google search and apparently "It is I" is the formal way to say it. But I pretty much never hear people use that in everyday life.

The short answer, for modern English, is "it's me". There's an old rule about a subject pronoun being able to follow "to be", in which case "it is I" is technically correct, but then again it sounds a little odd for a subject to follow the verb. I guess we've just gone with what's more comfortable. By the way, "it's I" is completely wrong. If you're going to be formal, you need to say every word fully.

It is I = old English It is me = modern English "It is I" is still used in some very formal situations. You won't be needing it, though.

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