Learning a language takes time and effort, especially if it is not your first language. Even if it is your mother tongue, you still need time and effort to master it because almost every language has its own slang and colloquial expressions, and the English language is no exception.
Language is forever changing. What is currently acceptable or popular may be replaced by something else in years to come, and the use of slang is a strong testament to that. Slang is just an alternative way of saying something. It is sometimes hard to identify what is slang and what is not. Slang and colloquial expressions are often acceptable in informal writing because they are used in communication in movies, newspapers, radio, television, and other mass media The more you learn, the more you will know when to use or not to use them in your formal writing. No matter what, knowing these common everyday expressions is a plus for all ESL learners.
Also, read my other book on American Idioms.