Firstly improve your vocabulary.A good writer has a good vocabulary — one that is both broad and deep — because a good vocabulary is essential to clarity, power, and precision. If you have a large stock of words, you can choose the most effective word. Secondly improve your spelling.Learning to spell is integral to good writing, because correct spelling ultimately helps you communicate your message to others. Since language is shared, then your spelling must follow the accepted standard pattern. On the other hand reading other people's writing also plays an important role in improving your own writing and good grammar prevents ambiguity.By contrast,bad grammar confuses your reader,slows their reading and shows your ignorance,which can lose you respect,influence,and credibility.

The best way is to read a lot of books (start with children's books) and watch movies in English ;) Also listening to an audiobook while reading the same book. And do some exercises. Here are some links: http://www.ego4u.com/en/cram-up/tests ; http://www.british-study.com/quizzes/grammar-test.php ; http://a4esl.org/a/g.html . Just search for "english grammar test" and you will find many more! Good luck.