Start with a brain storm.
Some things to help:
-why you are interested in the profession
-experience you have in the profession
-why the specific school you are applying to interests you
-your professional and personal goals
-how the school will help you receive your goals
after brainstorming, start to piece together the information into an essay.
Once you have a solid draft, I would suggest handing it over to a professor, SLP, or experienced writer to look over it.