I wrote up a list of qualities that I thought were important and that I thoght were my strengths. Then I thought of good stories that would highlight those qualities and figured out which story would be the most interesting/stand out.
Unless you have a really good reason to, I wouldn't write about the law just thinking that's what they want to hear. I have a friend who wrote about how he chose law after dealing with a friends rape and that was apropos. But too many people write same old same old "the law is important blah blah blah."
Also, they want to know your writing skills so make sure your essay is tight - perfect spelling, perfect grammar, etc. Make sure it flows, etc.