Try and write about something that showcases your strengths and the qualities that would make you a good physician. It doesn't have to be technical. Maybe something that shows how you are a leader. Or a story from your past that demonstrates your ability to remain calm under pressure.
Avoid the sick family member cliche unless it is a very unique one. Don't overly embellish unless you are good at it. And never start your statement with "I've always wanted to become a doctor since..." or the like.