Establish your proper business format. This can look several different ways, but above all it should be clean, easy to read, and professional. Include a heading with your name, address, email and telephone number.
Address the letter. On the top lefthand corner of the letter, you should have a section with the date, "attn" (Name(s) to whom the letter is directed) and "re" (subject matter of the letter). These preface the letter and clarify any questions that the reader might have about who you are writing to and why.
Begin your letter. If you can, obtain the name of the individual to whom you are writing. This will create a more personal effect, far superior to "Dear Sir or Madam," or "To Whom it May Concern."
State your reason for writing within the first paragraph. Be creative and personable in introducing yourself, but don't forget your reason for writing, and don't let your reader forget, either.
Establish your credentials. Your second paragraph should state why you are not just qualified, but ideal for the opportunity at hand. List your most relevant skills and remember to include something unique about yourself that other applicants will not have.
In conclusion, reiterate what you have said in a new way. Remind your reader of what an interesting, unique candidate you are, why you are qualified and why you are pursuing this opportunity. Thank them for their time. Conclude in such a way that shows that you are confident in yourself, and the reader should be, too.