The secret to being a good writer is being a reader.
There. I’ve said it. You can all go out and become the next J.K. Rowlings. Go on now, get it over with.
Okay, maybe it isn’t quite so easy, but it is the starting point. You don’t only have to be a reader, but a good one too. You should read anything and everything you can get your hands on if you want to be a writer. It’s not only enjoyable, but it familiarizes you with language, how it flows, how it intrinsically works. Maybe you speak English perfectly well already, but there’s a difference between spoken word and that which is written.
It’s difficult to explain until it is experienced. All I can suggest is to simply read. Read for fun. Read for depth. Read for dialogue, for characters, for plot, for deeper meaning, for grammar, for knowledge, for the love of it.
However...I will leave you with another secret.
The secret to writing...is that there is no one secret to writing.
To be a good writer, one must not only be a good reader, but also be willing to do something that might sound all too preposterous at first. A writer must be willing to write. And write. And write. And write. And write. Practice doesn't exactly make perfect, but it does constitute improvement, and that is all a writer can hope for.