Dear (use last name of contact here),
Paragraph 1: State why you are writing, which will include stating the full title of the job that you are applying for. You also need to say why the interviewer should select you for interview, and why you feel that you are suitable for the job.
Paragraph 2: Here you can explain what you offer to the company. You can discuss qualifications that are relevant to the post, and previous job experience which is relevant.
Paragraph 3: This should be a concluding paragraph. Here you can reiterate your desire to work in this position, mention again that you think you are an excellent candidate and give very brief reasons for this, and do not forget to thank your prospective employer in advance for considering you for the post.