Review the criteria set forth by your desired college for personal statements (some use these instead of, or in combination with, personal essays). Choose from their list of topics, if they have one. Choose the third or fourth one down on the list -- most high school students will write about the top one or two, and you want yours to seem original.
Be specific in your writing. List detailed examples of why you are a great fit for this college, what you have brought to educational institutions (such as high school) in the past, and what you are prepared to bring to theirs. Avoid being vague in your statement, and saying things like "I've always been a good student," and "I love to learn." Think of what distinguishes you from the rest of the student population, and write about that.
Provide supporting evidence for your statement. Think of the statement as a persuasive essay -- you are trying to convince the admissions board to let you in the door. Write the statement in typical persuasive format: introduction, body and conclusion. Reiterate in your conclusion why you are the right fit for their college.
Edit your personal statement -- never send in your first draft. Check carefully for weak arguments, rambling (admissions boards want essays that are concise -- if there's a word or page limit, don't go over it) and spelling or grammar errors. Have someone else read your statement over as well. Let them catch mistakes you may have missed.
Create an original personal statement. Be aware that many colleges and universities now have software that can check to see if a statement has been plagiarized or purchased off the Internet.