It may have been a difficult decision to quit your job, but writing the resignation letter shouldn't be. Resignation letters are brief and concisely explain your decision to leave the job. The best resignation letters are gracious and avoid accusatory language.
Format the letter like a formal business letter. Add the current date at the top of the page. Skip two lines and type your boss's name, company name and business address.
Address the letter to your boss in a friendly, and personal manner.
The body of your letter should state when you plan to officially leave your job. Give an exact date for your last day of work. Provide a quick explanation for why you're leaving such as getting a new job or leaving for graduate school. Keep it positive. Don't go into a diatribe about how awful working conditions are -- you're leaving for a new opportunity. Thank your boss for the opportunity to work with her and the company.
Finish your letter by leaving a forwarding address so that you can keep in contact with the company and so that the company will know where to send any important information or documents. End with "Sincerely" and sign your name. You should send a copy of the letter to the human resources department for your personnel file.