As a teacher I have found that writing has never been an easy task for students. Even if English is your mother tongue you will still have some difficulties & a loss of words.Writing should be taken into account on the basis of your vocabulary & sentences with grammatical errors. Read a few articles from an English newspaper/magazine/ internet and write a similar column. Ask some one like your teacher to read it & check it. Then try again and see if you can improve yourself. Always keep a dictionary with you or if you use a computer to type, use synonym tools. Have a notepad beside you when you look at the Italki questions and answers to jot down vocab. Then write another article a few days later and give to a friend/teacher after you think you have improved. Italki Resources have some helpful exercises for you to complete.

By reading a lot. Yes, I mean really a lot. This applies to any language.