Do you mind if I correct your post?
Hi. I would like to master English. What do I have to do?
Hi. I want to master English. What do I have to do?
Mastering any language is not easy. I would say to never stop learning. Many people study for years and believe that they are fluent and continue to make errors. Try to imitate educated native speakers. Learn slang as someone suggested but do not use it unless you are certain that it is the right situation. Read a lot and pay attention when you are reading to spelling, grammar, word choice, punctuation, etc. Do not just read to know the story. In fact, it may be a good idea to read something where you already know what happens. Try a book that you know well from your own language.