One way to improve your English is to learn English idioms - and then use them. Idioms add interest to what you say or write, and they make you sound more like a native speaker.
Take, for example, the idiom "water baby". This is someone (often a child) who loves being in the water. When you hear or see the idiom, you get the idea of a child who sees the water almost like a second home. So you could say to someone "Her son loves being in the water" or you could use the idiom and say "Her son is such a water baby".
Like other languages, English has thousands of idioms which you can use in many different situations, or to talk about your feelings and opinions. So how do you learn them?
1. Learn idioms in context or by theme
It's difficult to learn vocabulary through lists. Instead, make sure you understand when you can use a particular idiom by learning it in context or by theme.
For example, on this site we have an Idioms section which is separated into different themed pages.
Make sure you have an example of how the idiom is used, whether it's common or old-fashioned (English-speaking people rarely say "It's raining cats and dogs," for example) and in which situations you'll hear it. This is important, because if you use the idiom in the wrong situation, it won't sound natural.
2. Don't try to learn too many at once
Idioms can be complicated. For that reason, don't try to learn more than five at any one time. Don't forget: you'll need to practise your new idioms (like any new vocabulary) so limit what you try to do in any one study session.
3. Understand the feeling
We often use an idiom to convey a feeling or emotion. So for example, we say that someone has a "heart of gold". Because gold is a precious metal, we can imagine that someone with a heart of gold is a good person. (In fact, it means that a person is very kind.)
Idioms can also give you an image or a mental picture. For example, imagine that a person (Joe) is "under someone's thumb". You get the mental picture that Joe is ruled or controlled by the other person.
4. Listen out for idioms
If you hear two words used together in an unusual way, it might well be a new idiom. Some of the time you can guess the meaning through understanding the feeling and the situation, but you might also need to make a note of it, or ask the person who has spoken it.
You can also check the meaning of idioms in the Free Dictionary.