Do all you can to get the individual's name to whom your letter should be addressed. If you can't identify the individual, try the person's job title or department. Begin your research by calling the company's main number. Even “Dear Hiring Manager” is better than “To Whom It May Concern”.
Your first paragraph should be a brief introduction that quickly sparks the employer’s interest and convincingly identifies why you believe you're a good fit for the position advertised.
You can use the next one or two paragraphs, the body of your letter, to match your skills to the stated requirements in the job description and to explain how your accomplishments and achievements support the claim you made in paragraph one. You should also show knowledge of the company here, and relate your capabilities back to what the organization would need.
Your closing paragraph should always include a thank you, your contact information, and a specific date you'll contact them about the next step in the process.
Review the application letter samples for different closings.
Keep in mind that brevity is as important as specificity, so even if you're writing two paragraphs, neither should be more than three sentences, and above all else, your document should never be more than one page.