"To master something" means to learn or understand it completely.
So mastering the English language is really a gradual process.
It begins with some vocabulary
and some basic
grammar rules. That's the basis, and once you have it you can
continue progressing.
However, and many English learners have had to face this truth, just studying English is
not enough. You have to actually practice it in the real
world. You have to use it, learn
from your mistakes and gain
confidence.
(By the way, spoken English can be quite different from written English. You can read about it here: Learn Spoken English)
So how do you do that? Do you move to an English speaking country?
Well, if you can, that might be the best option. If not, there are
other things you can do.
You can practice your English with a native English speaker. You can
exchange e-mails, do a text chat, or even a voice chat (which is
probably the hardest, but also the most rewarding).
Recently, I've found a website that lets you find pen pals/chat
partners for free! Isn't that cool? :)