College admissions officers read hundreds of essays. While your transcript and test scores tell them about your academic abilities, your essay is where you get to really share who you are with them. They'll be looking for a couple of main things, including a demonstration of your writing abilities, but more importantly, they want to know what you have to offer their college community.
Rather than writing to impress or sound smart, write about something that is meaningful to you, whether it be a challenge you've faced, a topic that you are passionate about, or your vision for your future. You have a unique perspective, background, and personality, and the best thing you can do is share that uniqueness in your essay.
See this page for a list of essay prompts for the Common Application for college admissions and the JoCo and OSAC scholarship applications.
College Essay Guy
As the name suggests, College Essay Guy is a go-to resource for all things related to college essays. You can use this workbook to help walk you through the process of deciding upon a topic and writing your essay. There are also tools such as this Essence Objects exercise and this Feelings and Needs exercise to help you brainstorm your essay.
Khan Academy
Khan Academy offers a series of videos that will guide you through the process of writing a strong admissions essay. You can watch the first video embedded on this page, or check out the whole series on Khan Academy.
BigFuture
Big Future's college essays page has lots of tips for writing personal essays, as well as videos from college admissions staff from around the country sharing their expertise.