How to speak good English?

I am not a native English speaker.I speak English but it is not fluent,nor do I sound good while speaking English.
How can I make myself sound good while speaking in English? I am not much concerned about accents,American or British accent,It should just sound good.
And how can I be more fluent in it? Because I think in english ,then convert it into English and then speak.English words don’t come to my mind instantly.
And what are some good English slangs,or idioms? How to remember amazing and interesting English words?