Write about what you know. If the school gives you a topic to write about, relate it to your life and personalize it. If you have an open-ended essay choice, then you should write about something that you are passionate about and that is important to you. Make sure to give your essay a unique spin on the topic; for example if you are going to write about being the star in your school's spring musical, write about how you became involved in theater and what it means to you.
Create a certain tone that you will carry throughout the essay. It is important to project a sincere tone with your essay because you want it to accurately reflect you as an individual. Use humor tastefully; add it in only if it will contribute to your essay in a positive way.
Check over your essay for grammar, spelling errors and content. Have at least two people read over your essay that you trust will give you constructive criticism and correct careless mistakes.