There is a very good (and short) book called The Elements of Style which might help you a lot. ie=UTF8&me=&qid=1385539256&seller=&sr=1-3
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It's not just a matter of writing good English.
You need to consider how to organize and format an essay. Have a look at this link.
Be careful to stick to the topic.
An essay that is written in perfect English but is poorly organized and off