Idioms with TELL

I tell you what
used when you are suggesting or offering something
I tell you what, let’s have a picnic this weekend.

I’m telling you
used to emphasize that what you are saying is true
I’m telling you, that’s how it happened.

I told you so
used to say that you were right about something
“It didn’t work.” “See. I told you so!”

to tell you the truth
used to say what you really think or feel
To tell you the truth, I’m completely bored.

tell me about it
used to say that you feel the same way or have had the same
experience
“I’ve been so tired lately.” “Yeah, tell me about it!”