Research law careers and decide what exactly what it is you want to do with your law career. Law school admissions departments will appreciate your knowledge and want to know that you are aware of what will be required of you in law school.
Consider what in your life has affected you the most. Sharing one or two experiences works best in your essay; more than that will make your essay weak. Try to use a more recent experience if possible, but if all other experiences are overshadowed by one in your childhood, use the most powerful one.
Decide how your experience will differentiate you from the other applicants. Remember you are competing against many other students who want to be lawyers, so your experience is what will bring you notice rather than your certainty you will be a good student and lawyer.
Write a draft essay that has an introduction, topic paragraphs and a conclusion. You want your essay to prove a point, which is how your experience has prepared you for law school and being a lawyer. You will need approximately two pages in the end, but you should start with more material than that in your first draft.
Include in your essay some information about how you have affected organizations you have been a part of and any abilities you have for organization and teamwork.
Edit your first draft through successive drafts until you have the approximate two pages that best reflect your goals and your writing skills. The admissions office will be evaluating the way you write as well.