The same question can be asked about the quickest way to improve any other language; your own language, Vietnamese, for example. The same principles apply.
Living in an English-speaking country which provides the environment and the exposure helps a lot.
Well, it basically boils down to two factors - practice and exposure.
P.S. I notice that you live in Australia. That is excellent opportunity for you to learn the language. People who live in Asian countries are not as lucky as you are.
not a teacher
Student or Learner