Write a "hook" or first paragraph that describes how you became interested in that subject. The "hook" should be a way of grabbing the reader's attention. It could be a personal anecdote or narrative. It should not be too long.
Describe ways in which you are well prepared to pursue postgraduate studies. Detail achievements in related courses.
List other achievements in extra curricular or co-curricular activities. If you have had a break in studies or poor grades, explain the circumstances.
Explain why you are interested in that particular research area. This is important as the admission committee needs to know how committed you are to research. Go to the department's website and look at the different research fields. Do not mention a research area that is not offered at that university. Mention the professors with whom you would like to work with.
Describe how you will use postgraduate studies in the future. This should tie in with your career goals.
Proofread your statement before submitting it.