When you are sifting through the vast amount of sample you will have access to, be sure that you are looking for samples that will pertain to you and your personal situation. Once you find sample personal statements that you think have helpful information you can print them out and highlight any areas that you feel may help you when you set out to write your own statement. It is also a good idea to take notes as you go along.
So how do I know what universities want? Start by reading about the university, the course you want to study and the "entry profiles" as these will give you information about the type of student the university is looking for. Universities also want the truth so do not lie about your abilities and they want your writing so do not ask someone to write the personal statement for you. Some areas to cover include why you want to study this subject, what your future career plans are, what you do in your free time, what skills you have (academic and non_academic), what's interesting and/ or different about you.