A few tips :-)
Begin early! You should start developing a personal statement and trying out topic ideas. Exact essay topics can be downloaded when college applications come out mid-summer, but the questions often remain the same for several years.
Unless the college specifically asks for it, avoid the general autobiographical essay. Most essay topics are fairly open-ended, but if the question is specific, be sure you answer it.
Try to avoid much-used topics (unless they are addressed in a unique way). Some examples include Outward Bound type experiences, trips that awakened you to difference and inequality, how sports builds character, etc. On the other hand, don't go overboard in your attempts to be original, memorable, or profound.
Avoid using the essay to apologize for some perceived inadequacy on your part. However, if there have been unusual circumstances in your life, the essay might be a good chance to explain them without making excuses.
Avoid attempts at humor that are obscure or not funny or experimental or stream-of-consciousness writing that may be confusing to the reader.
Avoid anything that may appear too offbeat, cute, or elitist.
Get feedback, a lot. Have a teacher, parents, friends, or neighbors read and critique your essay for its form and content. Does your essay effectively communicate who you are? Is the voice natural? Listen to their advice, but make the decisions that are right for you.
Your Personal Statement/Essay should sound like it was written by you and only you.
Spellcheck is not always your friend. It will not pick up homonyms (there/they’re/their), nor will it save you from accidentally sending your Vanderbilt essay to Emory.