But it also depends where you apply to as other people will have similar grades to you, they want to know why they should pick you over someone else but also it depends how early they get it so they can actually read your PS as to a late entry then they'll mostly look at your grades. Type Personal Statement Tips/sample on goole it comes up with stuff i did it awhile ago.
But basic guideline:
1. Why you want the choosen subject
2. Intrests/hobbies
3. Work Experince/ Vol work
4. Round it off