Just because you are attending an online university doesn't mean you are exempt from the age-old admissions essay. Don't sweat it - write about the subject you know best: yourself.
What makes you who you are? Any interesting hobbies or goals? Introduce yourself in a professional, yet conversational manner. This will help you demonstrate your intelligence as well as your individuality.
Make your essay stand out from the rest with a few fun facts. Keep the font and paper a professional standard and always proofread for errors.