You should take some time to plan the structure of the essay before you start. I usually write the main body of an essay first and then when it is finished I write the introduction and conclusion. I find that easier but everyone's different. And try to stick to the point. :)

although i'm not good at writing essay, i think u should write more and more and have sb find your mistakes. You also read more samples and learn from them. Just what i think and learn form my teachers ^^