The word "personal" should say it all. This essay is about you, and you need to show what you have done and where you plan to end up in your life. If you have not figured that out yet, state what interests you plan on pursuing. Colleges are looking for what makes someone unique and how those traits will apply to your time as a student.
When writing a personal statement, it is a good idea to ask the following: Where do I wish to end up? Perhaps you wish to be an award-winning scientist or become a teacher. A strong personal statement lists what goals you wish to achieve, in both your academic and future career, and how you plan to achieve them.
If you are planning on applying to a program that is offering limited spaces, you can only assume that everyone applying will have just as long a list of accomplishments as you. That is why it is important to set yourself apart from the pack and really sell why you should be in that particular program and what makes you stand out from the rest. Draw from your life experiences, and state how this will drive you to succeed.
Once you have finished writing, do not assume that you are already done and ready to send. You could have thought of the most beautiful story in the history of mankind, but if your personal statement is full of typos and grammatical errors, then you are giving the impression that you really did not care to take the time to craft the best statement possible. Revise it until you are satisfied that you have written the best personal statement you are capable of.