Read the application and any instructions given on it. Most college applications will pose questions that should be answered within the body of your personal statement. Ignoring these questions is a cardinal error, although it may seem easier to send the same personal statement to several colleges. Ensure that you read what information the college is asking for, and give it to them. For example, UC Davis asks how your school, community or family has shaped your dreams and aspirations and how a personal quality, talent or accomplishment that you have makes you proud.
Write about why you want to do that specific course. Even though it may not really be the case, colleges want to know that you want to go to their institution and take their course specifically. Talk about what draws you to the specific course of study and the specific location in which you wish to do it.
Mention any accomplishments you have in academic or extracurricular activities. You want to show your college that you are a step above most applicants, and show them how much effort you put into your chosen field. If you have accomplished anything in the same field as your desired course, mention that, but if you haven't, point out the general positive qualities you needed to achieve the accomplishment. For example, getting a gold medal in the 100-meter sprint may be useless to the field of biology, but it shows that you are determined and put effort into a task.
Ensure that your personal statement is structured correctly. You may have a plethora of experience and achievements in your desired field, but if you don't know how to arrange them in your personal statement, it could appear disorganized. Structure your essay logically, starting with your general goals, moving on to what motivates you for the course, and then why you think you are suitable for it. Ensure that each point leads from the previous one and that you explain why any accomplishments are relevant to your course of study.
Ask a parent or teacher to read your personal statement. This can help give you feedback in case something isn't clear or doesn't come across well, and gives you an additional pair of eyes to spot grammatical or spelling errors. Get them to give you honest feedback, and then build what they suggest into your final draft.