Since writing is thinking on paper, really good writers are those who have a command of the language (ample vocabularies and an understanding of a variety of sentence structures). So, you will want to read, read, read so that you become acquainted with what good writing is. Then, reflect upon and ponder what you have written. Become analytical, for a good thinker is an interesting writer.
On the advice from the editor above, work on perfecting the basic unit, the paragraph. There is a site below on how to write a perfect paragraph. See the essay lab (www.enotes.com/essay/group) also, as well as the sites listed below.
As far as how to start, there is no one right way. In a recent video made by writers for Time magazine, each person had a different method. One, for instance, straightens his desk--this activity helps him think. Another writes ideas down on a yellow pad. Yet another makes lists of ideas after he has read copiously on the topic assigned. Still another freewrites and talks to others about her topic.
You will have to try a few different methods until you find your own way. But practice does make improvements, and enotes "how-to" topics will help you.
Main ingredients of a good essay or any other piece of writing can be broadly classified in two categories - the content and the language. Content refers to the information, thoughts and logic presented, and language represents the way this content is presented.
To be able to write good essays you need two different types of skills to take care of these two aspect of essays. You must have the ability choose and develop appropriate contents. Also you should have the ability to express yourself clearly and interestingly. Though while writing an essay, you will need to use both these skill interactively, it is possible to practice and develop these skills independently. When you do this your task of learning to write essays will become easier.
Another hint to make the task of learning to write good essays is to start with small essays on subjects that you know well. As a part of teaching effective business communication to post-graduate students in management, I ask my students to write 100 word write ups on different subjects, and my students have reported back that they find it very useful in improving their ability to express themselves clearly and persuasively.
Frequently what appears to you a difficult essay can be broken up in several smaller sub-sections. So, when you develop the ability to write small and simple essays, you are already well on your way to writing good long essays.