How do I start my personal statement letter?

Write everything that you know about yourself. How detailed or polished it is, that doesn't matter yet. Why? It's easier to trim down and polish than to come up with the perfect phrasing from scratch.

After that, write down everything that you like about the degree that you're applying to. Example: if you're applying to biomedical sciences, write down why you think biology is attractive as a research field. Sub-example: "There is no more interdisciplinary field than biomedical sciences: one has to have basic knowledge in chemistry, physics, statistics and even computer science in today's data driven approaches. That is why I believe I am a good candidate to become a student at your University."