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Personal pronoun

Personal pronoun

Hi to everyone!
I wonder what are the cases when I should use the personal pronoun "Me" instead of "I". For example, "It's me" not "It's I". It's difficult for me to understand. I could only try to learn these cases by heart.
The other case. For example "I have a photo album with pictures of me". Is there a big difference when I say my friend and friend of mine?

Could anybody explain to me this?

Thanks in advance.