Why not do a search first? :) http://voxy.com/blog/index.php/2012/02/top-10-most-common-idioms-in-english/ I personally don't agree with the list, but it's a start.

Lots of them. It depends on the point you touch upon. Weather , work ... e.g. It's raining cats and dogs - means it is raining a lot (heavy rain)

Thank you Dylan!

Hello Tetê, here are some common idioms that I know: "Never judge a book by its cover" - which means that there may be more to someone/something than it seems on the surface. "Piece of cake" - something is easy. "Break a leg" - Using this actually means good luck! (used in mostly in theater) Hope this helps!