I hope you would like to go to law school because you have a passion for law, and if that is the case, that passion should drive your essay. Your personal essay should be a good balance of the personal and the visionary. Here are a few questions that might guide you.
When and how did you realize that you wanted to become an attorney? Is there an anecdote about this that is worth sharing? What activities have you participated in that advanced your understanding of the law and its importance? How do you see law being used to sort out people's rights and duties? What skills and interests do you have that you believe would make you succeed at law? What is it about law that you believe you would find challenging and satisfying? Do you want to use law to solve societal problems? Do you want use law to right wrongs? Do you want to use law to empower the disenfranchised? Are you awed by how eight pages (the Constitution) have provided the underpinnings for our entire system of law? Are you interested in today's global village, in which international law is becoming increasingly important?
Under no circumstances should you indicate that you want to become a lawyer because it is lucrative or because you didn't know what else to do with your undergraduate degree, even if either is the case. Something worthwhile has made you consider this path, and that is what you need to focus on.
Given that the top law programs have far more applicants than they can admit, and that many of those applicants have high GPAs and LSAT scores, a good personal essay can make a difference between admission and rejection.
As with any essay, you should be careful to edit for spelling and grammar, make sure you have a clear focus and organization, and strive for a clear and interesting prose style. Because law requires very precise use of language, you should be especially careful about word choice.
Next, you should not use a single generic essay for all the schools to which you are applying. Instead, you should research each school carefully, and show in your essay precisely why you would be a good fit for that school. You should focus on one area of law in which you wish to specialize and then show how the school's offerings and staff in that area would be a good fit for your aspirations.
You want to include one or two personal anecdotes to give the admissions staff a sense of your character, but they should be chosen to illustrate why you would do well at or be committed to the branch of law you plan to study, rather than being chosen at random.