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.
As mentioned, this part of your application can be used to decide whether a school wants to invite you for an interview or not. Schools only accept students who have been invited for interviews, so getting to this step is a huge part of the process.