I had a little trouble too. This might help: Think of a couple of situations in your life that really stand out in your memory. This could be a graduation, death, birth, or just a moment that you shared with someone special. Whatever. Make a list of those moments and then think about how they have affected the person you have become and the choices you make now. Not all of them will have affected you all that much, but I'd be willing to bet that there's at least ONE that has had an impact on you.
Not all of us can be first-gen immigrants who had to learn English and take care of our sick families while working five jobs and maintaining straight A's. If you fall into a more vanilla category of applicant, just think of why and how you ended up at this point in your life and write about that. Just be sure to keep it personal and NOT re-state your resume. I hope this helps, I know how much this stage of the process sucks, I almost worried myself into a nut house. Best of luck!