I'm not a native speaker. When I'm writing or speaking my own language. I know how to use it in a proper way. For example, I know how to be polite when speaking or writing to a person I respect or I'm asking for help; I also know how to talk or write to my friends in a casual way so that they won't think I'm dull.
:wink: BUT I have no idea about how to do that in English. :? Can you help me?
That's what languages are all about. All too often you will just sound like "a nice guy" when speaking in another language, b/c that's all you've been taught. Read comics, watch TV and movies without reading the subtitles, hang out in your local pub, listen to the songs' lyrics, read English books if you enjoy it, participate in forums on the Internet, or, if you like to mix business and pleasure and you want real fast results: find an english-speaking girlfriend
If written English is your concern, my advice is that you should read as much as possible. It's a widely known fact that learners who read a lot are usually better writers (they master a better style, make fewer mistakes, have a command of broader vocabulary) than those who disregard reading. The above statement fully applies to the process of learning one's native language, too.
The pieces of advice the others have just given you are perfectly valid, of course.
Did you try getting a book about maneurisms? If it's all about being nice, then songs or TV, won't do it! Slang... Offensive and many other factors might mislead you.
How to write or speak in a proper way? 
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