"I'll tell you what" is slang, or a short form for "I'll tell you what you want to know (even if you don't want to know it!)"
"I'll tell you something" is pretty much the same thing.
I'll tell you what: I'll go to the store if you start the laundry.
I'll tell you what: Keep your nose out of my business or you'll be sorry!
I'll tell you something: That is the heaviest rain I've ever been caught in!
I'll tell you something: I don't like what I hear about him!
It's an interjection - something must always follow it. Sometimes it just means, "Wow!" sometimes "Okay" and sometimes it's a warning: "Watch it, buster."
- For Teachers