The college application essay should not be used as a place to repeat all of your accomplishments, test scores, and grades. Any information that is already in your application shouldn't be reiterated in the essay portion.
The basic purpose of the essay is to let the admission committee see who you are as a person. It's okay to say a few words about why you want to attend that specific college, but don't spend the entire essay "sucking up."
The essay needs to be focused on how your life experiences have changed you for the better. Most college applications will have another place where weaknesses can be discussed, but the essay isn't the place for it. Instead, point out the strengths in your character and how those strengths would benefit the college you are applying to.
Proofread your essay several times and have another person check it as well. Any spelling and punctuation errors will detract from the substance of your words. Plus, since you are trying to persuade an educational institution to admit you, these mistakes might make them think twice about your abilities.