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.
Remember, your personal statement is 'personal' in nature so it is in your best interest to be sure that all the information you provide is in fact your information. By starting with one sample and taking some ideas, then moving on to another and taking additional ideas, and so on, you allow your true self to come out in the finished product and this will give you the best shot at gaining access to the school of your choice.