"Je te dis."
Translation:I am telling you.
I assume that say or speak can't be used here because they both require inserting to for the sentence to work.
"to" or "that" but in English, you may keep it as is, while in French it really lacks an object.
I am thinking in particular about a phrase I like a lot: "I am not asking you, I am telling you!" (ce n'est pas une question, c'est un ordre)