Dear (Mr./Mrs./Miss/Ms.) followed by last name, (only use To Whom It May Concern if you have exhausted all attempts to find the name of the person to direct your letter to)
In your introductory paragraph, explain right away that you are interested in the internship position, and where you saw it listed. If there is no advertised position, simply state that you are interested in working as an intern.
The main body of your internship letter should express your interest in the company, why you feel you would be a good fit, and any skills or qualifications that make you a good candidate for the internship. Write with enthusiasm, and explain how the internship would benefit both you and the company. Explain how you could expand your skills for future employment, and how your assistance would help the company in some way.
In closing, inform the recipient that you desire an interview at their convenience, and that you will call in a few days to follow up on your internship letter. Thank them for their time and consideration.
Your name (first and last)
Sign your letter directly above your typed signature. As you can see, writing an internship letter is not such a difficult task. Follow the tips and guidelines above, and you will create a winning letter that is professional and makes an impact on the recipient!