For example, when someone says 'It's useless to beat a dead horse' or 'Don't beat a dead horse', it means that you shouldn't do something that will not yield any results. It is just like saying 'pull up socks' which means that you should get ready for a situation.
'Take it with a pinch of salt' is an expression used when someone makes outlandish or bogus claims. Like if a person says something that can't be entirely correct, you tell others to take the person's statement with a pinch of salt.
Similarly, if you don't want someone to tell a secret to others, you can say, 'Don't spill the beans'. About someone who doesn't hesitate in telling truth even in difficult situations when it takes courage or others may feel offended, you can say, 'He calls a spade a spade'.
When you want to say that you will scold someone, you can say 'I will take him to task', or 'I will give him a dressing down'. If you know entire details about a project or you can tell numbers, you can say, 'I have it on my fingertips'.
These are just some examples. Earlier also posts on Idioms and Phrases have been published on this blog. The aim is to give you an understanding about phrases and the fact that you should develop an interest in learning more phrases and retaining them so that you can use them in your conversation.
Find more phrases, learn them through your books and also check in dictionaries. Just be alert when someone uses a phrase. Hopefully you won't get flummoxed and understand the difference between literal and real meaning.