Try to use English as much as possible. Try to read English books, watch English t.v. shows, and listen to English music. You can also try speaking it with a native English speaker as well. The more you use it, the better you will become.

Like Britta said, watch English TV shows and listen to English music. Find stuff you like on youtube, and watch/listen to it over and over again. Look up words you don't know. Don't be afraid to make mistakes!

What stage you mean , i use english i wotk all the time wiyh my manager and in my reports

Practice, practice and more practice

Depends what stage you are now, I guess.