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Best way is go to Japan and work there.
listen and watch Japanese drama as much as u can .
otherwise u can start studying online or at the nearest Japanese teaching center.
If you are not anymore in the beginner's level, READ A LOT.
And read out loud.
So that you can get used to the different structure of the language and you can feel the sense of which particle to use at the right time. Of course you can learn Kanji at the same time.
The easiest way to learn any foreign language is to be in this particular language environment, where all surrounded people are native speakers and using just this language in their daily life. And also a person should exclude the usage of his/her native language.