Keep your mouth shut. 

跟任何人说都不行。/这是秘密。

英英释义：

This means "Don't tell anyone." It is said when there is a secret to be kept.

Keep this to yourself. 

这是秘密。/你一个人知道就行了。/别对任何人讲。

英英释义：

This usually means something is private.It is like saying "Please don't tell anyone else."

The cat's out of the bag. 

秘密被揭穿了。 

英英释义： 

This indicates "Someone told the secret." Often it is a way to say now many people know the secret.