Mehdi, it may be too late to add major accomplishments, but framing the accomplishments you have is another story! So, review your activities over the last 4 or 5 years and make sure you haven't forgotten any. Its not just what you did in school (clubs, activities and leadership positions), but on the weekends (youth counselor at church), and summers (various jobs). Did you travel and have unusual experiences? What else filled your high school years? List everything to help as you write about yourself.
Ask a favorite teacher to help remind you of what makes you unique, and use that to further frame your experiences. What you've learned from your activities can be the basis for a compelling essay ("how having the job no one else wanted made me more motivated", for example). Its up to you to present yourself as someone who would add to the student body and to the college. Remember that your scores and your grades will help you in the door, but its how you describe yourself and your experiences that will compel a school to offer you a spot.
Best of luck!
Ilene recommends the following next steps:
Please use personal experiences to your essay. I mean, if that is actually a requirement. Essays were necessary when I went to college and am not sure if they are still relevant.