First, start with the basics. A cover letter should have the date written and the name and address of the person you are writing to on the left side of the page. Some people choose to put their own name and address on the right side or top left side of the page. Begin your letter with "Dear Mr./Ms. [Last Name]." Be sure to sign the end of the letter with "Sincerely, [Your Name]." Also, be sure to mention the title or job you are applying for in your letter.
A cover letter should highlight your skills and experience for a particular job or type of job. Someone in high school will most likely have a limited amount of work experience. If you have no work experience, then highlight skills you have from any volunteer work, school activities, clubs, extracurricular activities, etc. For example, if you are applying for a job at a school or day care center, you might highlight experience that you have as a baby-sitter. If you are applying for a job at a restaurant, you could mention that you were in a high school cooking class. Be sure to explain your experience in complete sentences. List responsibilities you had or skills you learned at any previous job or during any other activities. Please remember that a well written letter that has been carefully proofread will be taken more seriously. A poorly written letter might reflect negatively on you.