If you didn't study law, don't worry that your qualifications or motivation are being questioned - law firms are very interested in candidates studying other subjects and value what they can bring to the firm. And if you studied law, you'll still have to explain why, even if it's simply because you wanted a head start on a legal career.

Whatever degree you did, try to demonstrate there was logical thinking behind your decision, outline what skills relevant to a legal career your degree has given you, and show a bit of your personality by conveying what it is about the subject that interests you.