It’s okay to want to talk about an obstacle or challenge you have faced in your life, but DON'T dwell on telling the story of this challenge – talk about what you learned from it, why you are a better person for having gone through it and how you’ve grown it.
Nobody is going to learn anything of value from you if you fill your essay with complaints, excuses and self-loathing.
Having a mentor or guide who understands the writing process is invaluable.
This doesn’t mean that they’re doing the writing for you! This is a lot harder than writing about the War of 1812.
“I want to cure cancer and find a solution to our energy crisis”) or wherein your minor extracurriculars are overly-inflated (“my one semester as vice secretary for the Student World Affairs Club changed my life” or “the two hours I volunteered at the soup kitchen really changed lives”).
Be specific and be realistic: don’t say “I want to work with children,” if you can say “I hope to increase my volunteer efforts with after school programs and lobby for more funding for Teach for America.” Qualify your accomplishments into realizable chunks: “I am new to the Student World Affairs Club, but I hope to make its newsletters more effective.” Rather than make these two common application mistakes, instead prefer specificity to vagueness, and a realistic portrayal of your current place in life: as a capable but eager-to-learn young adult getting ready to move into the world.
You are neither a grandiose giant nor a silly baby, so don't portray yourself as one!
DON'T tell the reader what they already know about you.
One thing you absolutely should DO is read your essay out loud to yourself. To see if your voice and your personality are really on that piece of paper.
Are you in that essay or does it just sound like it could be anyone else? Very few people can do that well, and it’s easy to come off as arrogant, obnoxious or shallow. College admissions officers can sniff this out in a second. That happens when you talk about how you act, respond, think and feel against the backdrop of your topic. The people reading your essays have been through emotional and challenging experiences of their own.