Best way to improve your writing in English is practice. Practice reading the writing of others and study their sentence structure and vocabulary. Look up any words unfamiliar to you. Identify the parts of a sentence and punctuation used.
Also, practice your own writing. If you read a news story in your local paper, try to translate it into English and rewrite your English version. Keep a daily diary and write about your day in English.
Try to learn a new word each day and practice using it in a sentence that shows you understand the meaning of the word.
Even native speakers learn use of the language by practicing, rewriting and refining their writing.