Moved this to the Personal Statement Advice forum.
Yeah it's hard to start one, I remember cringing at every sentence I wrote initially. Two main themes to mention are why you chose the subject and how you demonstrate your interest in it. To actually get started, maybe try just blurting out everything you could possibly have to say about that, get all your ideas written down. Once you've done that it'll be much easier to pick out the best bits and see what to include and what doesn't need mentioning.