So do I/ Neither do I
'So do I' is used to say that a positive sentence is also true for me.
Paul: I like chocolate.
Mike: So do I.(= I also like chocolate)
'Neither do I' is used to say that a negative sentence is also true for me.
Paul: I don't like to go to school everyday.
Mike: neither do I.(= I don't like to go to school everyday)
Both are often used as a reply to someone else in a conversation and we can use both sentences
Mike:John has been to London, and so have I.
Mike:Lucy doesn't like fish. and neither do I.