Write in a natural style. Writing, like conversation, should flow naturally. Don't try to impress your reader with long words or complex sentences. Write so that readers will understand your ideas. Develop an ear for good writing style.
Good writing is more than following rules about grammar and punctuation.
To be a good writer, you need to be aware of style. You can develop this awareness for good writing style. Read a lot. Read about many different subjects. The more you read and write, the better you will be at it.