Dear Mr Pratt:
First Paragraph. Write an engaging, professional opening that conveys your personality and interest in them. Identify what you are applying for and how you learned of the opportunity.
Second Paragraph summarizes what you have to offer the employer in relation to their requirements. If you have a job description, you want to use the specific language of the job description when possible.
When possible, you should also include why you are specifically interested in working for that employer. If you have only one sentence to convey, this can be done as a part of either the second paragraph or the closing paragraph. However, if you have more to say, it can be another paragraph.
Closing Paragraph thanks the reader(s) for their time and consideration and lets them know what your next step will be (e.g., “I will be calling you in two weeks to see if an interview is appropriate,” or “I look forward to hearing from you soon so that we can further discuss my skills and qualifications.”)