"You're telling me" means that something is so clear that it doesn't need to be said. It can be either positive (I know, and I agree with you) or negative (I know, so you don't have to tell me). For example: - "Wow, that exam sure was hard! - You're telling me!" - "Darling, we're heading straight into a dead end. I think you should have turned left at the fork in the road. - You're telling me!" In both cases, "You're telling me!" could be replaced by "No kidding!"

Eh, it can be both, depending on your inflection and the subject you're talking about. The "I completely agree" version would be something like this: "Man, I hate when people throw trash on the sidewalk." "Yeah, you're telling me!" The "Duh, stop repeating this shit that I already know; you're an idiot" version would be something like this: "Did you know that the blue whale is the largest living animal on Earth?" "Pfft... You're telling ME?" Again, it's a matter of tone. If you emphasize the "me" and say it with a sarcastic or irritated tone of voice, you mean it in the second, arrogant sense. If you don't emphasize any of the three words and you say it with either a calm or an excited tone, then you mean it in the first, friendly sense. As for what else you can say to express that you're in complete agreement with something... Well, say, "I'm in complete agreement!" I think they'll understand that pretty well. ;)