Read what you wrote. Judge whether it sounds the way you think it should sound.
Read novels. Lots of novels. Any novels. Notice how they all have perfect grammar, perfect spelling, perfect layout. (Every dialog gets a new paragraph, every paragraph preceded by a blank line, and so on.) Keep a notebook where you record figures of speech and anything you think is a good example of writing, along with the reference so you know where you got it.
It takes a lot of reading to be a good writer.