A cover letter is a business letter that is typically submitted with a resume as a way of introducing yourself to the person that does the hiring in an organisation or business. There is generally no difference between a cover letter for a data entry clerk job and most other jobs, except that you should clearly state your experience as a data entry clerk in your cover letter and that you're looking for a data-entry clerk job.
Address your cover letter to the person at the company who does the hiring for the data-entry clerk job that you desire.
Introduce yourself in the first paragraph and explain that you are applying for a data-entry clerk job. Mention where you saw the job advertised or who recommended that you apply.
Explain who you are and why you are qualified for the job. Add pertinent details, such as your education and previous employment in the data-entry field.
Mention that you are familiar with what the company does and what industry it is in. This will convey to the interviewer that you have researched the company and aren't sending out resumes randomly.
Close your letter by stating how you meet the data-entry clerk requirements that the company is looking for and that you are enclosing your resume with the cover letter.
Use a search engine on the Internet or call the actual company to research the name of the person that you will send your cover letter and resume to. Check for the correct spelling of the interviewer's name and what his title is (e.g. Mr., Mrs., Miss).