Determine exactly who or what you want to depict in your essay. Unless your assignment has specific guidelines, you are only limited only by your imagination.
Brainstorm the characteristics of your topic. These should include physical attributes and character qualities. Concentrate on using all of your senses to describe your subject. You might find a thesaurus helpful during this exercise.
Use the list you created to figure out which specific aspect of your topic you want to describe. It could your sister's genuine heart, or the way her hair shines like silk. You could even decide to write about your sister as a whole. It doesn't matter which quality you choose to be the focus of your essay, as long as you are passionate about sharing your topic with the reader.