Learn idioms and figures of speech. English is full of phrases that seem to make no sense unless you understand the meanings behind them. Some of them may seem to be "out in left field," but don't let them "throw you for a loop."
Know the differences between slang terms and proper ways of speaking. If you are talking to an average English speaker, you will likely use slang at some point, which include very informal words and phrases. Understand when to use more formal speech and when it is more appropriate to use informal words and phrases.
Practice using the idioms, figures of speech and slang terms you learn as much as possible. Join an online language exchange, such as MyLanguageExchange.com, so that you can use what you learn by speaking with a native English speaker who wants to learn your language.