There's so much to talk about, I don't know where to start.
Literal
Talking [nominalizer-こと] [subject-が] too-many [and so-て], where [from-から] start-talking [if-たら] good [question-か] don't-know.
~すぎて ('so much that...') sets up the consequence. ~たらいいか分からない is a set pattern for expressing indecision — literally 'I don't know whether it would be good if I started from where.' Natural and frequently heard when someone is overwhelmed with things to share.