I can think of three occasions, usually as "I'll tell you what" or just "tell you what". It could been used when wanting to end something, expressing a strong feeling: I'll tell you what, just do your thing and I'll do mine. I'll tell you what, keep your thoughts to yourself. or just a general suggestion: I'll tell you what, how about I pay this time and you get it next time? I'll tell you what, let me go first and I'll show you how it's done. or just to share what you think: I'll tell you what, I'm surprised you weren't grounded for being so late. I'll tell you what, I think Chelsea will win the league this year. Otherwise, it could be used in a sentence (no comma), such as: I'll tell you what he said. I'll tell you what you want to hear, but it's not the truth.

its "I tell you what", and it's american slang for "let me tell you something...." it's used to start a sentence