Take a sheet of paper. But the word "me" in the middle. Draw lines coming off to topics that are important to you. Family, interests, activities, beliefs, whatever. Add details to these topics. Decide which of these you'd like to write about.
You start to write an essay by releasing the Caps Lock on your machine. Then, make a list of the things that you believe are important about you. Your name, age and family background might be a place to start. Then add your interests, dislikes, strengths and weaknesses, etc. I think you'll surprise yourself as to how easy it is.