pronouns - Which one is correct to say: "It's me" or "It's I"?

I was taught at school that the following expression is not grammatically correct:

Who is there? It's me.

The correct one is:

Who is there? It's I.

Can you let me know which one is accurate?

Here is a good explanation about both forms.

Answer

As reported from the NOAD:

me /mi/
pronoun [first person singular]
1. used by a speaker to refer to himself or herself as the object of a verb or preposition:

Do you understand me?
Wait for me!

• used after the verb to be and after than or as:

Hi, it's me.
You have more than me.

• informal to or for myself:

I've got me a job.

It's then correct to say it's me.