Any language on earth contains idiomatic expressions, proverbs or general sayings that are unique and meant to be understood figuratively to convey a strong message. When learning an additional language, many students focus a lot on vocabulary, grammar and pronunciation, but sometimes don't really put a lot of effort into learning colloquialisms. That is a mistake, and here's why.
Idioms are part of what makes a language rich and beautiful. They are also a recognition of the fact that us humans communicate on a higher level than other species, as opposed to dogs, for example. We use phrases and expressions that have a different meaning than that of the actual words, incorporating humor and different effects such as emphasis. Understanding and using this form of communication is intellectually stimulating and refreshing, reminding us that languages are by no means boring.
Another reason is the fact that, if you strive for excellence and want to reach fluency, you will simply have to learn and use even the more "complex" aspects of a language. This will allow you to start sounding like a native speaker, and if that is your goal, using idioms will be a big leap forward.
How can you learn figurative expressions? The most fun and authentic way would be to interact with a native speaker as often as possible and observe the way these expressions present themselves in a natural way. When they use an expression you don't understand, ask them to explain the meaning to you and try to remember it so you can use it in the future.
Another way is to watch a lot of (good quality) English movies or series, as well as reading (books, blogs, articles, whatever). These will not only improve your knowledge of idioms, but also your vocabulary.
The last and admittedly least fun way is by consulting textbooks and websites that will list English idioms and their meanings. These could be very helpful since you will be able to learn them on your own time, have them ready to memorize and you will probably understand them better if the origin, meaning and examples are explained to you by academic professionals.
If you think it might be difficult, don't be discouraged. Once you start learning them, you will soon find it interesting and entertaining! Now, grab the bull by the horns and good luck!