tell me about it!

Tell me about it! AND Tell me! (=I know, I agree with you, l understand what you are saying — I've had the same experience, or I've had this problem too.) — И не говори! Кому ты говоришь, (у самого так же было.)

Said in reaction to someone else's statement, as in the examples
below

Example 1: I love my sister, but she can be a real bother sometimes. — Oh, tell me about it. (thefreedictionary.com)

Example 2: It's a nightmare, isn't it? — Oh, tell me (about it)! I don't know how you managed it.

Example 3: We had a hard time finding the place. — Tell me (about it)! It took me all morning. (The American Heritage)

Note
Identical to a literal request to be told about something, this expression must be distinguished from it by the context and the speaker's tone. (The American Heritage)

see also:
[you can say that again]
[you're telling me]
[ditto]
[same here]
and how!
you bet!
righto
you said it