The most efficient way to learn is to actually speak it. Maybe correspond with someone overseas who speaks English and don't speak Indonesian, or if they do, at least it's minimal, so that way you are forced to write/speak fully in English. It doesn't matter if your grammar is all over the place to start with, because the main thing is that you're trying.
My other trick is to watch a lot of English speaking movies/tv series without subtitle, or set the subtitle in English. That way you'll get the grasp on jokes/slang/every day conversations, and you'll have a better idea on how the conversations work.
And last but not least, read a lot. I started out with a lighter reading (e.g. fiction in the genre you enjoy). I'm pretty sure the first English novel I ever bought and read was a novel by Christopher Pike (can't remember what the title was, but I'll know it if I see the cover). Have a dictionary close by as you'll need it if you're starting out.
