One of the rubbish things about being a native English-speaker, therefore, is that it's actually quite difficult to get a good grounding in why we say the things we do. For example: when to use I and when to use me.
I is a subjective form (I'm following him - I'm doing the action in this sentence, so I'm the subject)
Me is an objective form (He's following me - He's doing the action in this sentence, so I'm the object)
Grammatically, it's not possible to have an objective form of the verb to be. This means the answer to Who's there? should always be It is I. But would we really say that nowadays? Wouldn't it just sound archaic and pedantic? Wouldn't we, instead, say It's me, even though it's wrong?
God, I'd love it if we did, in fact, say IT IS I! in daily life. Sigh. Mind you, I'm still upset that we don't say "Zounds" any longer, so am perhaps not the best person to give advice in this matter.
English is always evolving and doesn't stand still. What's unacceptable for one generation is completely natural for another. I still shudder when I hear people use hopefully to mean I hope that will be the case, but understand that the usage has reached a momentum that can't be stopped. Bah!
A couple of times when it's obligatory, even now, to use I and not me:
Following as and than when writing: Emma is faster than I (although you'd say Emma is faster than me).
Following as and than and then continuing the sentence: Emma is faster than I am and has more hair (subjective form obligatory for both written and spoken forms).
Have me explained this well enough?