A personal statement is a statement of your ideas and goals.  Some programs request one or two paragraphs, while others require longer essays in which you will write at length about your preparation and motivation for graduate study.  Before writing, consider what your reader might be looking for.  Study the program website.

There are two main approaches to organizing an essay.  You can outline the points you want to cover and then expand on them, or you can put your ideas down on paper as they come to you, going over them, eliminating some, and moving others around until you achieve a logical sequence.  Making an outline will probably lead to a well-organized essay, whereas writing spontaneously may yield a more inspired piece of writing.  You choose the approach.

If you need to explain anything that could be construed as negative in your application (low grades or low test scores, for example), you can do this briefly on a separate sheet of paper titled "Addendum" which you will submit separately.  Do not address this in the statement; keep the statement positive.