Your best bet is to tell a story about yourself. Now to the mechanics (in descending order of importance): The first rule of good writing, fiction or non-fiction, (whichever you choose to describe yourself) is SHOW, DON'T TELL. If a conversation with your grandmother taught you something important, just recount what was said. The "moral" of the story is implicit and should be left for the reader to discern without being "told" what he or she has learned.
Another requirement for good writing is rewriting. Unless you are a Mozart of the keyboard and are "taking dictation from God," your work will benefit from rewriting - several times.
Have your essay reviewed by a competent writer. (This may or may not include your high school English teacher. If in doubt, ask to see a writing sample from her.) Comments on mechanics and content are useful. It is important, however, that the editing not submerge your authentic "voice" which is, after all, what the colleges are interested in. If your essay sounds like it was written by your uncle, the novelist,