Read a book once a week, and write an essay summarizing the book afterwards. Pay attention to the following details:
Where: the place and background of the story.
When: the time of the story (modern or historical).
Why: the purpose of the story.
What: the happenings of the story.
How: the progression of the story.
Ask yourself why the author wrote the way it is. If you were the author, would you write differently and how.
Describe to others what the most exciting/worst parts are in the story, and why others should read or buy the book.
Tell others what lessons you have learned from the story.
After a few weeks, you will surely become a good essay-writer.