The key is to start writing your personal statement quite early on, as most people don't get to their final statement until they've written a few drafts. Generally, your statement should be divided up into a few sections.
This needs to be something that grabs the readers attention, as they have to go through hundreds of these statements. Try to express an original and honest reason behind what drew you towards medicine.
2. Work Experience
Talk about what you learned being a doctor is like from your personal experience at hospitals/surgeries of anywhere else you've worked. From this, dissect the arguments for and against being a doctor and explain how the pros outweight the cons for you. Talk about the various skills you noticed were important for a doctor to have and say why. You could compare various healthcare settings if you have worked in more than one e.g. a hospital and a GP surgery.
3. Voluntary Work
Talk about what you've done and learnt and how you can apply skills gained from this to medical school and being a doctor. This includes mentoring students or other positions of responsibility at school such as being a prefect. Talk about a few skills, 3/4 at most.
4. Extracurricular Activities
This is your chance to show the reader what you enjoy apart from medicine. You can talk about sports/music/books/debating among other things and explain how you find these activities useful e.g. stress relief when workload is high.
(5. Gap Year)
If you are a gap year student, talk about what you plan on doing in your year out and how this will benefit you.
Summarise why you want to pursue medicine as a career and what makes you right for it! Don't add in any new information at this point.
Make sure you check all your punctuation and grammar before submitting! This is just a general outline of what your statement could be like, everyone has different ideas and you can always incorporate them in and rearrange the order of paragraphs to make your statement 'personal'.