as easy as pie: very easy.

Ex: The question is as easy as a pie.

cost (someone) an arm and a leg: something very expensiveannnd cost a lot.

Ex: The computer cost me an arm and a leg.

tell a white lie: You do not want to hurt someone so you say something that is not true in order.

Ex: My cousin drawed a picture today. It was terrible. But I don't want to hurt him so I told a white lie, I said the picture was beautiful.

(fromhttp://www.eslcafe.com/idioms/id-mngs.html" title="http://www.eslcafe.com/idioms/id-mngs.html" target="_blank"http://www.eslcafe.com/idioms...)

let the cat out of the bag: share secret.

Ex: "I tell you don't let anyone know but you let the cat out of the bag."

(from http://www.goenglish.com/LetTheCatOutOfTheBag.asp" title="http://www.goenglish.com/LetTheCatOutOfTheBag.asp" target="_blank"http://www.goenglish.com/LetT...)

get cold feet: Suddenly become very nervous about doing something that you were going to do.

EX:I always get cold feet when I am taking an exam.  

(from http://humanities.byu.edu/elc/student/idioms/id iomsmain.html" title="http://humanities.byu.edu/elc/student/idioms/id iomsmain.html" target="_blank"http://humanities.byu.edu/elc...)