This is some pretty basic info that anyone can tell you...I don't know what you're writing about, but I hope it helps:
1. Hopefully you have something that makes you interesting. At the very least, make your first few lines unique. "My path to medicine is a long one...blah blah blah" is like an auto-snooze.
2. Tell your story about how you reached medicine. Was there a situation in your life that sparked your interest? How did you develop your desire to pursue this career? This could be an individual situation, or a build-up of experiences over time.
3. Don't say the same thing twice in a different way.
4. Don't repeat yourself. You'll sound dumb. (see what I just did?) It's easy to do if you're trying to fill your character count.
5. Tie your thoughts together with your final paragraph.
6. Please proof-read. Typos are bad in a PS.