This is a question I should know the answer to, but for some reason, I don't. People in my class write their dialogues like this: ["Carrie," I said, "What was my sister's name?"] Is that comma after 'said' supposed to be there, or should it be a period?
No, the comma is correct.
If the words in the first set of quotations marks (before I said) were a complete sentence, then you'd need a period after said.