There are some decent guidelines on the UCAS web site..have a look.
Are you a mature student or in 6th form? If in 6th form your tutor will read the draft and send you away with it many times..
If not..as Pipsy suggests.. write down a few headings and jot down EVERYTHING that occurs to you ..do that before you think of starting.
You have 4000 characters inc spaces.. 47 lines in total. So you need to be succinct but at the same time convey a real enthusiasm. I would suggest opening with a WHY.. there must be something that sparked off your first interest.. an event in your life, an experience, something like that.
How your A levels are relevant if they are/you are doing them (only brief but shows a link)
Then your relevant experience.. not what you did as much as what your learned from it ('I worked in X as a volunteer and learned how to adapt my communications skills... bla bla)
Anything else that makes you fab.. hobbies etc..but don't make them up if they don't exist! Work experience otherwise.. you can develop a lot of people skills in the most mundane jobs!
What you expect to gain from uni.. and a last line summing up what qualities you have that will make you the person they want.
Sanam... if you can't be helpful, why post?! Personal Statements are not easy and if you have never done one before , hard to get started. Not knowing how to start a PS is NOT the same as not really wanting the job. So you aren't being helpful in any way shape or form.
I have proof read rather a lot over the last few years and most students need a fair few attempts to get their heads round them.. and the hardest bit is starting.
Good luck OP, and if you need help proof reading PM me:)