Understand what the statement is requesting. Are you being asked about your strengths and weaknesses? Is it a general personal statement? Once you have established what the admissions committee is looking for, the next step is to break down your response. Answering the question becomes dramatically easier once you have a plan.
Your statement should focus on three areas: achievements, experiences and future ambitions. Start with explaining past achievements and awards that have gotten you to your present state: perhaps a research award, academic recognition or professional relationship piqued your interest in your intended field of study.
Experiences should be non-academic (i.e., not in the classroom) and contextualize your ability to be a successful and contributing member of the academic department. Discuss research experience you may have had, extra-curricular activities, travel or volunteer opportunities that have established and strengthened positive character traits.
The third part of a successful personal statement is a motivated outlook towards the future. Future ambitions should be clear, concise and reasonably focused. You should explain how graduate school will help you contribute to society, your academic field or your intended career.
Don't worry if you are uncertain or if you think things might change in the future. The admissions committee is looking more for forethought about your commitment to graduate school than a hard-and-fast plan. Part of pursuing education is figuring out your place in the world. An applicant with self-assurance and direction comes across at motivated and a certain asset to the department.