From Longman Dictionary of Contemporary EnglishI’ll tell you something/one thing/another thingI’ll tell you something/one thing/another thing (also let me tell you something/one thing/another thing)OPINION used to emphasize what you are saying I’ll tell you one thing – you’ll never get me to vote for him. Let me tell you something – if I catch you kids smoking again, you’ll be grounded for a month at least. → tell
Examples from the Corpus
I’ll tell you something/one thing/another thing• Let me tell you something - if I catch you kids smoking, you'll be grounded for a whole year at least. 